Colchester manager Joe Dunne was angry with referee Trevor Kettle after seeing his side beaten 3-1 by Coventry.
The Sky Blues were already 1-0 up through Franck Moussa's strike when William Edjenguele's header was controversially allowed to stand, despite the assistant referee initially flagging Gary McSheffrey offside.
Tom Eastman gave Colchester hope with 19 minutes to go but in-form David McGoldrick wrapped up the points for Coventry late on.
Dunne said: "I've spoken to the referee and the explanation he gave me is incredible. There's a guy attacking the ball (Gary McSheffrey) who's going into my goalkeeper and in an offside position.
"How can you not be interfering with play or attack a ball in an offside position and not be offside? It was incredible.
"I believe Coventry's second goal was offside and he (the assistant referee) has put the flag up straight away. I think the assistant overruled himself with his flag still in the air.
"I always made a promise to myself as a manager to never get aggressive with officials and I think that they have a hard job.
"But we've had a couple of calls now in the last three games that have helped in us not being able to win the game.
"It's very disappointing because I thought up until the first goal, we were well in the game."
Coventry boss Mark Robins said: "We played really well and I thought we were outstanding. We passed the ball really and it was an outstanding team performance."