The midfielder has recovered from a hamstring strain sustained in the League Cup semi-final with Liverpool, and got 20 minutes under his belt against Everton last week.
But the Spaniard is fighting the flu ahead of Chelsea's trip to Paris and Mourinho is concerned that the 27-year-old will be unable to start against the Ligue 1 champions, admitting that the side struggle in his absence.
“We have only little problems, not everybody is perfect,” said Mourinho. “Oscar has a problem, Fabregas was ill, Willian still has a little feeling from the Liverpool game, but everybody wanted to try and I would say that only Mikel is completely out.
“We are a different team with Fabregas, I’m not saying better or worse, I’m saying different. The team grew up from pre-season with him on the team and the team went in a certain direction with a style of play.
“Without him, we lose some control, some possession, we become the same team, well organised, good in transition. But with him, we control the game better, we have more control of the game. We have possession of the ball because of the nature of the style of player he is.”