The Swede was left out of the squad for Saturday's win over Arsenal.
But manager Jose Mourinho insists the move is precautionary and not because Ibrahimovic is injured.
"Zlatan is not injured. Zlatan sometimes needs very hard work on the muscular area that supports the knee that had the problem," Mourinho said.
"When the period for that hard work comes, also the fatigue comes so he cannot play for the se days.
"So we decided to do that after Watford. He will be out again for tomorrow but I believe he will be back to normal work after CSKA."