Bent out of Netherlands friendly
Bent rolled his ankle after a challenge from Antolin Alcaraz in the 0-0 draw at the DW Stadium and left the ground wearing a protective boot.
McLeish said he feared Bent could now face a significant spell on the sidelines, and said Wigan's pitch had contributed to the incident.
"Going by his reaction, it seems serious," said McLeish, who faces a problem up front as Robbie Keane is due to end his loan spell and return to Los Angeles Galaxy.
"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."
England play the Netherlands at Wembley on Wednesday.