Jose Mourinho said Eden Hazard must have a "serious injury" after he came off in Chelsea's 2-1 defeat at Leicester but said he did not know the nature of the problem.
Hazard limped out of the match at the King Power Stadium on 31 minutes after a late challenge by Jamie Vardy and was replaced by Pedro Rodriguez.
It is unclear how long the Belgium international is likely to be sidelined, though, with Mourinho telling reporters: "I don't know what is wrong.
"The only thing I know is in 10 seconds he made the decision himself. It must be a serious injury because he just left the pitch and just made the decision not to go in.
"His first reaction was 'I can't' and I was going to make the change. His second reaction was 'I want to try,' and when he goes on the pitch, first step he immediately says 'I can't do it.' It was him in a few seconds who had three different perspectives, but clearly he couldn't do it."
Mourinho, who accused his players of betraying his work with their performance, praised Leicester for their victory but also hit out at the ball boys at the King Power Stadium.
"They [Leicester] fought with everything, they defended with everything, and the ball boys were amazing too," he said.
"I say the point about the ball boys because it's a disgrace for the Premier League, but I want to make it clear I don't want that story to be in front of a result Leicester deserved."
It is not long until the January transfer window but that is far from Mourinho's mind, with the manager insisting it is up to the current group of players to turn things around starting with the pair of home games coming up
Mourinho was also asked whether Chelsea would be looking to reinforce in the January window, but he said: "I don't think it is right I go to the board and say these players are not good and we need to spend 50 or 100 or whatever.
"We have these players and it is with these players we have to go, and the players that are not performing well must feel attacked in their pride, in their self-esteem, and they have to do everything to get results in a humble way.
"I don't think they can feel they are top players or they are superstars. At the moment they have to be humble, they have to look to the Leicester boys and feel that they are the stars, they are the top players.
"They have to look to Sunderland and Watford and say: we are at the same level as you are. The players have to put their feet on their ground and be humble. We are where Sunderland are; we are below Watford by 10 points. The next two matches we play at Stamford Bridge we have to look to these opponents in a humble way."