Chris Lynn and Sunil Narine have opened for the men in purple since Lynn has been fit to play after being out for a month with shoulder injury.
But Narine faced just four balls chasing 174, with Tim Southee taking the West Indian's wicket for a duck.
"It's the formula we have followed throughout the tournament. It was about Narine and Lynn trying to put pressure on the opposition," Boult said at the post-match press conference on Saturday night.
KKR made a hash of their run chase with none of the batsmen being able to deliver.
Manish Pandey (33) was the highest scorer and looked the only sensible player on the pitch as the hosts threw the game away after being 53/3 at the end of powerplay overs.
"The powerplay is a big phase of the game. It has come off very well for us in other games. I don't think there's any need to change the formula. It was just unfortunate that it did not work out tonight," Boult said.
"In this format, momentum is a big thing obviously. You would like to win as many games as you can. Unfortunately, it did not pan out our way," the New Zealand pacer said.
"We had hurdles in the last couple of games. We know we have to play good cricket from now on. Every game is a tough game. It's a must-win game for us now. The pressure is on us to deliver. Unfortunately we ran out of luck," he added.
Boult said the wicket was two-paced and the opposition team bowled really well.
"There was a little bit of variation and bounce. The luxury of bowling second is you can put the pressure on the opposition, claim wickets and make it very hard for the team to chase a decent total," said the New Zealander.
"We probably did not bat well enough, first of all. But I thought they bowled really well on a two-paced wicket."
Mumbai rested six of their first team players.
"It's disappointing to lose no matter which team it is.
"It's a quality opposition and they can afford the luxury of moving a few players around and try a few combination. They delivered under pressure," Boult said.
Boult's previous match in this IPL was against Sunrisers Hyderabad on April 15 and he was the pick of KKR bowlers then with 2/30 from his full quota of four overs.
"I was nice to be off few weeks. It's hard to push into the side with only four having the chance to make the playing eleven. I enjoyed been out there tonight and contributing to the side. We have a lot of quality in the group."