Bent left the DW Stadium in a protective boot after landing awkwardly following a challenge from Antolin Alcaraz, and McLeish believes there may be ligament damage.
"He was in agony in the dressing room," McLeish said. "Going by his reaction, it seems serious. You never know if it settles down in the next 24 to 48 hours before he has a scan, but he went over on it badly.
"The pitch was very heavy because of the rugby. It sapped the energy from a lot of players and Darren has been unlucky to go into one of the pieces of turf that has been dug up.
"Normally it is your ligaments that are affected in a situation like that. He has been grimacing and has been a little bit nauseous in the dressing room."
McLeish said there was no question of Bent featuring for England in the friendly with Netherlands on Wednesday, and said he did not expect the Football Association to insist on carrying out its own fitness assessment.
"He is certainly out of the England squad," McLeish said. "I think there will be a bit of common sense applied to this one.
"Our doctor has a good relationship with the England medical set-up. If he has to report he will certainly be limping into the headquarters."