Fabregas started on the bench against Everton in the Premier League last week after hurting his hamstring against Liverpool in the Capital One Cup at the end of January, but the Spaniard is now struggling with the flu.
At Chelsea's final training session in London before their match in Paris on Tuesday, Fabregas was fully covered, including a woolly hat on his head and snood around his neck.
Mourinho insisted it is just a "little problem" but it remains unclear how Chelsea's midfield will look against PSG.
"We have only little problems, not everybody is perfect," Chelsea's manager said.
"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 [Jon Obi] 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."
