Fellaini is a divisive figure whose midfield presence was missed in Saturday's 2-2 draw at Stoke in the Premier League, but he is back in the squad for tonight's Champions League clash.
The re-occurrence of a calf complaint sustained on international duty was behind his absence and appeared to grate on Mourinho as Fellaini played Belgium's match against Greece despite the issue rearing its head beforehand.
The 29-year-old missed training on Sunday but returned to the group on Monday, when Marcos Rojo, Luke Shaw and Ashley Young were among others in action on their return from long-term injuries.
"He's a very important player for me - much more important than what you can imagine," Mourinho said. "I feel weaker without Fellaini in my squad.
"It doesn't matter if it is on the pitch or if it is on the bench, so if his conditions improved he will be selected because I need him. But in this moment I cannot say yet."