Manuel Pellegrini bemoaned Stuart Attwell's decision to award Manchester City a contentious penalty as West Ham were defeated 1-0 at the Etihad.
The Chilean was critical of the match official's decision to penalise Felipe Anderson for a foul on Bernardo Silva after 59 minutes of resolute defending from the visitors on Wednesday night.
Manuel Lanzini had opted not to go down under a challenge from Nicolas Otamendi nine minutes earlier, but Sergio Aguero made no mistake from the spot to keep City's title hopes on track.
Pellegrini said: "We lost to a penalty that was not a penalty. In the other box, maybe if Lanzini goes to the floor then maybe he can whistle. They had other chances after the penalty, but we ultimately lost to it so I'm unhappy.
"Apart from the first 10 minutes, we defended well. I think if you review the match, Lukasz Fabianski only made one or two saves. I think the penalty was the first risk that he had on his goal."
West Ham remain in 10th place - four points adrift of Wolves in seventh and a potential European spot.
Indeed, this was the first time in the Premier League this season that City have been restricted to just one goal in a home game.
Pep Guardiola's side have kept up the pace with Liverpool, and had Andy Carroll been more clinical with his one chance in front of goal, the visitors may have provided a major dent in City's title aspirations.
The hosts had 76 per cent possession and 20 shots, but it was not until after Aguero's contentious spot-kick that they came close to hitting their high standards - to West Ham's credit.
Pellegrini added: "We defended well but we didn't look after the ball well enough. Considering that we have eight players injured, and four players that we rested, of course I can't be very unhappy with the result.
"We had the first clear chance [Andy Carroll's shot], but I think the decisive moment was the penalty. We know they have a lot of players that can make the difference, but we worked well and our intention was to recover the ball and have speed in front.
"Unfortunately, Marko Arnautovic woke up this morning sick and couldn't play, but I thought we defended well."