Kolkata, Dec 19 (IANS): Reigning champions Mumbai Indians owner Akash Ambani on Thursday conceded that releasing as many as 12 players was the most difficult part ahead of Thursday's auction.

MI, which has won record four IPL titles, released 12 players and retained 18 which helped them win its fourth IPL title earlier this year. The likes of Australian fast bowler Jason Behrendorff and Alzarri Joseph of West Indies, New Zealand's Adam Milne, Beuran Hendricks and Ben Cutting were released.

Kiwi seamer Trent Boult, West Indian Sherfane Rutherford and Mumbai pacer Dhawal Kulkarni were the three trade-ins for the franchise.

"It's the most difficult part of IPL, especially as last year we were champions. We took some decisions, injuries were a big concern. We left and traded out 12 players this season, that's the most that MI has ever done in last 5-6 years. But they were the toughest decisions," Ambani told reporters at the media conference in between the auction.

Mumbai have a settled look about their squad and only needed back-ups coming into the auctions with seven (five domestic, two overseas) slots remaining.

The franchise grabbed Aussie pair of Nathan Coulter-Nile (Rs8 crore) and Chris Lynn (base price Rs2 crore) respectively besides bringing back Saurabh Tiwary (20 lakh).

"We had identified the seven slots that we wanted to fill. It was very critical for us to get Lynn and Coulter-Nile. Saurabh has been with us. We are happy with ours buys," Ambani added.