Herrera was passed fit to play in Spain, but he was replaced by Paul Pogba after only 17 minutes and it looks as if he could be spending more time on the sidelines.
"I think it is a bad injury," said Mourinho. "He had a small injury that stopped him for the last couple of matches but the medical team said he was 100 per cent ready for the last Saturday. It didn't look like he was fully fit."
The last time Herrera played was during the 2-0 loss to Tottenham at Wembley, but a hamstring problem has prevented any further appearances. Questions will undoubtedly be raised over his participation on Wednesday.