Thibaut Courtois admits he was surprised to see Eden Hazard substituted by Antonio Conte during the 2-1 defeat at Manchester United.
The Blues slipped out of the Premier League's top four after goals from Romelu Lukaku and Jesse Lingard earned United the points following Willian's opener.
Lingard's winner came two minutes after Hazard had been substituted by Conte, with Pedro coming on for the Belgium international.
Chelsea were then left without their most creative spark when chasing the game and Courtois has aired his surprise at his compatriot's withdrawal.
"I have no explanation for the substitution," Courtois told Belgian outlet Play Sports. "I did not expect it, but it’s the manager’s choice. He’ll explain it. I can’t look into his head.
"Of course, you want players like Eden 90 minutes on the pitch. He can always bring something extra."
Conte has been under pressure in recent weeks, with the Blues struggling domestically. Chelsea are currently fifth in the Premier League, two points behind fourth-placed Tottenham and 19 behind leaders Manchester City.
Speaking after the defeat at Old Trafford, Conte suggested that Hazard had failed to maintain his high standards in the second half and that his withdrawal was merely a tactical decision.
"No Hazard [is not injured], no," he told a news conference. "It was a tactical decision.
"You have to keep the same performance for 90 minutes, we didn't have a great balance, and every single player has to try to work with the ball, without the ball, otherwise we lose our balance.
"I think he'd finished his energy because the first half he ran a lot. The second half he started the same way, but when I see a player is a bit tired, my task is to find a different solution."
"After our goal, we conceded a counter-attacking goal just like last Tuesday but in the second half we had less of the ball, but even then we should never have lost this match," he said.
"If you do not do the small things well in top matches, you lose, which is what happened."