Rooney, 30, was due to start the Europa League Group A clash against Ukrainian side Zorya Luhansk after being dropped for United’s 4-1 win over Leicester on Saturday.
The Old Trafford boss said: “Wayne Rooney is selected (in the squad). I did not decide yet if I start with him.
“He didn’t train yesterday, he had a back problem. So today was the first time he had a training session with the other players and the intensity and complexity is obviously reducing because we have a game tomorrow.
“I was completely convinced of playing him tomorrow, but I am not sure if I do that because of the situation you created with him.
“He can’t afford to have a performance that is not really good. I am here to protect him and I have to analyse whether the best thing for him is to start the game if he is not totally ready.
“So I am going to discuss that with him and the medical staff and decide whether he starts or is on the bench.”