Barca host United defending a 1-0 lead for their quarterfinal tie.
"It could be that he starts and it could be that he doesn't," Valverde said. "He completed 60 minutes the other day [against Huesca], which was what was predicted more or less.
"The idea was that he'd be available.
"Those who start will be important, but also those that come on.
"United have something special in the last few minutes.
"In fact, they won the European Cup here in the last two minutes."