Ole Gunnar Solskjaer accepts he made a mistake in making two substitutions before the corner Leicester scored from to beat Manchester United.
Solskjaer withdrew Mason Greenwood and Anthony Elanga for Edinson Cavani and Marcus Rashford in the 65th minute and a minute later Leicester defender Caglar Soyuncu met Marc Albrighton's corner to net the eventual winner.
Rashford was tasked with marking Soyuncu, without a club goal in 18 months, and the Turk lost his marker to beat Nemanja Matic in the air.
Managers usually delay a substitution until after a corner they are defending is taken in order to avoid unsettling their set-up. United have a wretched record of conceding from set-pieces this season and Leicester's winner was the 10th goal they have given away from a corner this season.
"That's one of those golden rules isn't it, you don't make a change," Solskjaer conceded. "But, first of all, it wasn't a corner, it should have been throw-in and that's what bothers me more than anything else.
"It seems like I'm sulking again but when it's a throw-in it's a throw-in, it's not a corner. But we should have done better and disappointing when you lose a game on a set-play."
United are still two points clear with a game in hand on third place Leicester and can secure second with a win over Liverpool on Thursday.
Solskjaer made 10 changes for United's third Premier League game in five days against Leicester, with goalscorer Mason Greenwood the sole survivor from the weekend win over Aston Villa. Almost all of United's regulars are expected to be recalled for the visit of Liverpool, when United could all but end the deposed champions' lingering chances of a top four finish.
"After we lost a few points a few weeks ago, of course we set the target of getting to the final [of the Europa League] and being second and we know we need more points to get second position," Solskjaer added.
"So we need to make sure we're ready on Thursday, be ready for the challenge again, after that we've got two more games and hopefully get the points we need."
United's final fixtures are at home to Fulham next Tuesday and away at Wolves on Sunday week.