Rooney was dropped to the bench for Saturday's 4-1 win over Leicester City.
Mourinho stated at today's media conference: “Wayne Rooney is fit but I have not decided if I will start with him yet. He did not train yesterday. He did just a little bit of the session on Monday and he had a back problem.
"So today was the first time that he had training with the other players and today is the session where the intensity and the complexity is reduced because we have a game tomorrow.
"I was completely convinced of playing him, but with this situation in the last couple of days, if I do that, because of the situation you create with him. I think he really can't afford to have a performance that is not really good. And because I'm here to protect him and not to put him in difficult situations I have to analyse it."