The 22-year-old midfielder missed the draw against Norwich City after picking up a foot injury in training in the build up to the game.
McClaren has admitted that he doesn’t know how long the youngster will be out of action for, delivering another blow to the Rams as they fight for promotion into the Premier League.
“We don't know how long Omar will be out but it came as a big blow,” said McClaren.
Derby have been unfortunate with injuries in their push for promotion, as they have been missing other key players in key positions.
Chris Martin, Darren Bent and George Thorne are all examples of players who have been out of action during a crucial part of the side’s season.
They have a squad, which is able to cope without Mascarell, but it’s hardly an ideal situation as he’s been a consistent player in the middle of the team for most of the season.
What McClaren can’t do is fall into the trap of rushing him back into action prematurely, which risks keeping him out for an even longer period of time.