"The fact that they didn't have Robben in the side made a big, big difference," said City manager Kevin Keegan, whose side are the only team to have beaten Chelsea in the league, 1-0 in October.
Chelsea's lead in the standings was reduced to nine points over second-placed Manchester United with 12 games to go.
Robben suffered a foot injury just before scoring in Chelsea's 1-0 win at Blackburn Rovers on Wednesday and watched from the Stamford Bridge stands on Sunday.
Chelsea fear Robben's foot may be broken although they will not know the true extent of the damage until he has undergone a scan once the swelling has gone down.
"I just don't know," Mourinho repeated several times to persistent questions about the injury.
The versatile Robben, a roving winger, has been a key player in Chelsea's drive towards the Premier League title, scoring seven goals in 14 league games since recovering from a previous foot injury.
"I accept that Robben is a magnificent player and any team will miss a magnificent player," Mourinho said.
He was reluctant, however, to accept that the absence of Robben and striker Didier Drogba, who has scored eight league goals and is nursing a bruised thigh, had a bearing on the result.
"In terms of creativity and quality of play, I think so, but even without them we had enough chances to score goals and win the game," he said.
"So I prefer to say that my players in finishing situations were not lucky and in David James (City) had a goalkeeper who made some incredible saves."
