The Fall of the Mumbai Indians - Death of a Dynasty?

Jack Hope writes about a disappointing season for the Mumbai Indians and why it happened.

Going into the 2021 IPL season everyone was sure of one thing - Mumbai Indians were going to make the play-offs. They were back-to-back Champions, had beaten the second best team in the tournament four times last year, and with the emergence of stars like Ishan Kishan were not blighted by an ageing squad. 

We were happy to reaffirm this belief, heading into the second half of the competition, as Mumbai possessed a positive 4 - 3 record, despite having played the majority of their games at the Chepauk Stadium. Less than four weeks later they were knocked out of the tournament, as their so-so Net Run Rate combined with their 7 - 7 record to consign them to 5th place, behind a resurgent KKR.

How did this happen, and why?

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What is the problem?

Mumbai Indians started the 2021 IPL season as favourites for a reason: they were, by miles, the best team in the tournament over the preceding two years. 

To demonstrate how good we can take a look at a couple of radar charts, showing where they ranked in 5 key bowling, and 5 key batting metrics, across 2019 and 2020. The further each point of the pentagon is from the middle the better (if the point is furthest from the middle, that team ranked first in that metric, and obviously closest is worst).

These graphs don’t need an awful lot of commentary. The 2019-2020 Mumbai Indians were the best, or among the best, at everything they did. 

Now let’s look at the same charts for 2021.

This answers the “what happened?” part of this discussion. Simply put, their batting fell off a cliff. Alongside that, we also see a drop off on the bowling side, particularly in terms of their overall potency. However, their performance in that department would have seen them through to the play-offs if they’d batted with any consistency. 

*I’ve been asked why I use the metrics that I do, for the charts above. The first set of stats: Runs Per Ball, Average, Strike Rate, and Dismissal Rate, are all commonly used metrics that most fans will be familiar with, and tell us a lot about a team’s overall batting performance.

Then I’ve used Boundary %, as that tells us which teams are succeeding in the area that matters most in T20 cricket, with the vast majority of contests being won by the team that hits the most boundaries. 

Finally, are a couple of more contextual measures. 6:4 Ratio, which tells us literally how many sixes batters are hitting per 4, which gives us an insight into a player or players ability to continue to score quickly later in the innings, when more fielders can be pushed back onto the boundary. And finally Dot % gives us an insight into a team’s ability to build pressure. 

You will see that many of the stats above are strongly correlate with each other, but seen as a whole they provide a nice “at a glance” picture of a team’s performance versus its competition.

Why did it go wrong?

As we saw above, the story of the 2021 Mumbai Indians is a story about an elite side which forgot how to bat. Knowing this we can take a closer look at where it went wrong for their batters in 2021.

Coin tosses

The first thing to check, in terms of analysing where things went wrong for Mumbai, was their luck at the toss. In the second half of this year’s IPL, no fault can be found in Rohit Sharma’s luck at the toss, as he won 5 out of 7 tosses. They also had, what is generally considered to be, the advantage of batting second in 4 of the 7 matches (and batted first in their final game out of necessity).

Probably safe to rule this out as a major cause of their decline.

Venues

Next we turn to another factor outside of their control: venues. In this case Mumbai Indians were genuinely unlucky. Across the 6 venues they have played more than 2 matches at, between 2019 and 2021, they score slower than the league average at only one, the Chepauk Stadium in Chennai. Their next worst performance versus average is Dubai, where they score only slightly quicker than average. Across those two venues this season, their record was 2 wins and 5 defeats. 

At the two venues where they performed best versus league average, Delhi and the Sheikh Zyed Cricket Stadium, they won 4 games and lost just once.

There is obviously some noise in here, and it stands to reason that a team will have a better than average record at venues where it regularly wins. However, it isn’t outrageous to suggest that, had the tournament been played using at the same venues as 2020, then Mumbai may well have won the one additional game they needed to make the play-offs.

Variance

This brings us on to the most likely reason for the regression in Mumbai Indians batting this year: variance. Generally speaking this is signficantly underplayed as a factor in coverage of the IPL, and appears to have had a detrimental impact on Mumabi in two ways during 2021. Firstly in terms of how their entire season played out, and the combination of multiple players producing sub-par outputs, and secondly in terms of how individual matches sometimes span out of their control.

Seasonal Variance

Firstly, considering this from a big picture, we can roughly model the Mumbai Indians expected Runs Per Ball over a season by looking at historical data. This model is represented by the bell curve shown below, with the red lines annotating where the 2021 season sits on that curve.

Effectively, what this chart is telling us, is that we would expect Mumbai Indians to experience a season such as this, or worse, around 25% of the time. This is unfortunate for Mumbai Indians, but is well within the range of expected outcomes.

It is worth noting that this chart could be improved. Though it uses the real MI ball-by-ball data, it is largely unweighted, doesn’t take into account venues, and uses data from the whole three year range in question. Think of it as something that illustrates a real world phenomenon, and not something that you would bet your mortgage on!

As an extra step, I also looked into how opposition teams had changed which bowlers they used for each MI batter. I expected that, after two years of dominance, perhaps opposition Captains would have become more match-up savvy, and found ways to reduce MI’s edge. This, by and large, did not appear to be the case. 

In Match Variance

We also saw the impact of variance within individual games. Over a 14 match season it is perfectly plausible that the best team will not qualify because of the outsized impact of a few key moments. The perfect illustration of this came in the first game of the second half of the tournament, when Mumbai reduced CSK to a quivering wreck at the end of the powerplay - 4 wickets down, with another batter retired hurt - only to be on the receiving end of Ruturaj Gaikwad’s best career innings to that point. The Cricmetric win predictor is shown below, with CSK’s chance of victory, at one point falling to just 12%. 

Swings of this nature happen a lot in T20 cricket, and even teams as good as Mumbai are regularly on the receiving end of them. The main thing to learn from this is that if we want to see the best teams consistently make the play-offs then serious consideration should be given to extending the length of the season, to reduce the impact of variance.

It is also important to bear in mind that Mumbai do not have a monopoly on this kind of bad luck. Again, in a season where the finest of margins separated them from the play-offs, they really needed matches like the CSK one, to go their way.

So What Do We Learn?

It is a little bit of a cop-out to say that Mumbai Indians were just unlucky in 2021. However, the combination of venues, unexpected opposition performances, and widespread underperformance in their batting unit, were all factors that helped to torpedo them this year.

If we briefly consider what we would expect to happen if this season were replayed, I think most people would happily place money on Mumbai qualifying for the play-offs. This magnifies the point that we should be far more aware of the role of chance in the outcome of these short T20 tournaments.

I am also not saying that we should totally discount the possibility that some Mumbai Indians batters are worse now than they were two years ago, but we should be careful about leaping to that conclusion. And certainly, from a positive point of view, they still appear to have a lot of strong pieces they could retain going into this year’s mega-auction. The trickier questions they will need to answer are what to do with some of their elder statesmen, with emotion taken out of the equation, would it be sensible to retain Rohit Sharma?