The former Tottenham forward was joint-top scorer in last season's Barclays Premier League but has had fewer opportunies this term, despite scoring a glut of goals around the Christmas period and mustering nine in 18 league and cup games.
The manager is sympathetic to the striker's desire to play and will have more discussions at the end of the season.
"Dimitar could play tomorrow [in Bilbao] but I have good options with front players," stated Sir Alex. "His contract situation is we will be taking up the option on the contract but, having had chats with him, he wants to get first-team football and we need to consider that at the end of the season.
"At the moment, he remains at United. But I'd listen to what he says. I think he's concerned he isn't getting enough first-team football. It's difficult for me to guarantee that. At 31, you can understand he wants that. It may lead to him looking elsewhere."