The Frenchman was touch and go to start for United in their 2-1 win over Southampton but played almost the full game, pulling up in added time with a hamstring injury.
The winger himself described the problem as "bad” as he hobbled away from the United tunnel after the game – and Bruce admits he regrets picking Saint-Maximin just days after he suffered an injury against Sheffield United.
"We don't know how serious it is. He hadn't recovered but he is always a bit like that. I am kicking myself because if it is a long time... we hope it is not too serious," said Bruce.
"He has been crucial to us. He is a maverick free spirit. Wanted to play. When he pulls up the way he did, that's the thing you fear."