Every year the IPL throws up different types of challenges. The primary aim of all teams is to make the play-offs. In order to do that, they want to quickly identify their best XI early on. Ideally, the only changes expected should be tactical, based on the opposition and venue. At the halfway stage of IPL 2021, only a few teams have been able to achieve that. Following is the review of the changes teams have made, the reasons, and what we could expect in the second half of the tournament.  
PS: The piece has factored in team changes upto the 28th match of the season
Chennai Super Kings
Players used: 13
The players they've used is the least by any of the eight sides at the halfway mark. As captain, MS Dhoni's strength has been to give a long rope and tinkering only if absolutely necessary. Moeen Ali missed a game due to injury, which allowed Imran Tahir an opportunity to bolster the spin department on a slightly dry pitch in a day game in Mumbai. The other change has been Lungi Ngidi and Dwayne Bravo swapping positions based on the pitch and opposition.
Kolkata Knight Riders
Players used: 14
They haven't made sweeping changes, but they just have two wins so far. Having identified their best XI beforehand, the Knight Riders have backed their players who have failed to execute. Their changes have been purely from a venue or an opposition perspective. Sunil Narine replaced Shakib Al Hasan once they moved out of Chennai and Shivam Mavi came in for Harbhajan Singh in venues not favoring spin (Mumbai and Ahmedabad). With chances of qualifying for the play-offs fast shrinking, expect a major overhaul at the top of the order as well as some pace bowling reinforcement with Lockie Fergusson.
Royal Challengers Bangalore
Players used: 15
For the second season in a row, the Royals Challengers Bangalore have started well. There have been a few changes necessitated by the team playing at three venues. Some of these tweaks have been tactical. Rajat Patidar, Washington Sundar and Shahbaz Ahmed have all been rotated at No. 3 as per the match situation. Navdeep Saini and Kane Richardson played a match each in Mumbai where the pitch supports pace. In the last couple of matches, Daniel Sams replaced Daniel Christian as the overseas all-rounder. Expect the Royal Challengers to carry on with the same strategy.
Rajasthan Royals
Players used: 16
Ben Stokes, Jofra Archer, Liam Livingstone and Andrew Tye are out of the IPL, but the absence of any replacements has meant that the Royals have had to suffice with playing around with their Indian combinations. Legspinner Shreyas Gopal was drafted in the eleven against the Royal Challengers as a potential match up for Virat Kohli and AB De Villiers, but that did not work. Manan Vohra started as an opener, but failed, forcing the franchise to bring in the young and uncapped Yashasvi Jaiswal who has supported Jos Buttler. Kartik Tyagi missed the first six matches as he regaining fitness while Anuj Rawat were also given an opportunity.
Mumbai Indians
Players used 17
Traditionally Mumbai don't make too many changes. In IPL 2020, Mumbai used just 15 players - the least in the tournament. Already this season, the defending champions have fielded 17 players. Chris Lynn and Marco Jansen had to start due to the non-availability of Quinton de Kock and Nathan Coulter-Nile. Chennai, where Mumbai played their first set of matches, did not suit their batting style and hence they had to make a couple of tactical changes. Jayant Yadav played when there were a lot of left-handed batters in the opposition and if the pitches were slow and taking spin. But, in Delhi, playing against the Super Kings, Mumbai strengthened their pace attack playing Dhawal Kulkarni and Jimmy Neesham. It has meant that Ishan Kishan, who hit a staggering 30 sixes last IPL, has had to sit out after struggling for form in the intial matches this season, but also to accommodate the extra Indian bowler. Kishan's position has gone to Krunal Pandya, who has been given the new role of batting at No.4
Delhi Capitals
Players used 17
Delhi Capitals have mainly had to make a few forced changes. Indian bowling allrounder Axar Patel was indisposed recovering from Covid-19 and missed the first five matches. Patel's return offset the absence of R Ashwin, who was forced to leave the tournament after five matches to attend to his family that had been enveloped by Covid-19. South African fast bowling pair of Kagiso Rabada and Anrich Nortje joined the IPL late while Indian pair of Amit Mishra and Ishant Sharma missed the first few matches recovering from niggles. Tactically Steven Smith for Ajinkya Rahane was one change and the Capitals have made. Nortje has been kept out thanks to the scintillating form of uncapped Indian fast man Avesh Khan, who is the second-highest wicket-taker this IPL at the halfway stage. Don't fix what is not broken and expect the Capitals to persist with the winning combination.
Punjab Kings
Players used: 18
Similar to the Royal Challenges, the Punjab Kings have also played at three venues. That has prompted tactical changes. Despite enduring his worst IPL with four ducks in six matches, West Indies powerhitter Nicolas Pooran was persisted with for the first seven matches. The bowling department was changed based on the opposition and venue. One curious move by the Kings has been the late introduction of Ravi Bishnoi, who sparkled in IPL 2020 with his attitude and his googlies. Expect the Kings to continue backing their core group of batters with changes in their bowling line up as they move towards Bangalore for their second half, which is very batting friendly.
Sunrisers Hyderabad
Players used: 21
Sunrisers Hyderabad have had the worst start to the IPL since they entered the IPL in 2013. A solitary win in seven matches has meant they're at the bottom of the points table. Desperation to win combined with injuries to key bowlers has resulted in several changes - 21, the most for a team this tournament so far. The franchise also took the bold move of dropping all-time Sunrisers and IPL great David Warner as captain and player in order to provide more balance to the team. What has also not helped is lack of swing in venues like Chennai as well as the fitness struggles of Bhuvneshwar Kumar and the knee surgery that ruled out T Natarajan. The team that has made the IPL play-offs consistently since 2016 has pressed the panic button and expect that extending to the second half of the IPL.

Gaurav Sundararaman is a senior stats analyst at ESPNcricinfo