The 22-year-old had picked up a calf strain in the warm-up but felt he was still fit to start the game, only to be replaced after a quarter of the match.
However, Northampton director of rugby Mallinder is hopeful the injury is not serious and that the winger still has plenty to offer club and country this season.
"George North has a calf strain, hopefully it's not too bad," he said.
"He picked it up in the warm-up and thought he was OK to carry on but clearly wasn't 100 per cent.
"Tom Collins hasn't been selected for the England Under-20s so they must have some brilliant wingers. He's been great for us all season.
"Kahn Fotuali'i is a special player. There were two outstanding nines on the field with Danny Care for them, but I thought we marshalled him well."