Erik Ten Hag "can only pray" Rashford injury is not serious
Rashford, who has scored 28 goals for United in all competitions this season, grabbed the inside of his right leg after stretching for a high ball at Old Trafford and was replaced by Wout Weghorst for the final 10 minutes.
"Some things you can't afford," Ten Hag said.
"Why is the Premier League giving us the late Sunday night game and the early Saturday game? I think it's not right, and then you run the risk, players can't recover that quickly, and we know all the science, research gives you that players need a certain period to recover."
Saturday's match was United's third in seven days, and while Ten Hag said he was proud of his team's performances that have kept them ahead of Tottenham Hotspur in the top four, he was frustrated at the thought of losing his leading striker.
The schedule does not get any easier with a Europa League quarter-final, first leg on Thursday at home to Sevilla and a Premier League match on Sunday at Nottingham Forest.
"It's part of the schedule and now we are finding ourselves in this situation, and now we can only pray that (Rashford) is not dropping off (injured)," Ten Hag added.
The Dutchman did not elaborate on the extent of the injury.
"I can't say in the moment. Are you a doctor maybe?" he said with a laugh. "We have to wait for diagnosis, wait and see."
Rashford ran all over the pitch throughout United's dominant victory against Everton, assisting for Anthony Martial's 71st-minute goal that made it 2-0 while missing several excellent chances himself. But he paid for his busy afternoon and walked gingerly to the Old Trafford tunnel after exiting the game.
Ten Hag was asked why he did not take Rashford off after his team went 2-0 ahead.
"Fair question," the United manager answered, adding that he wanted to give Rashford and second-half substitute Martial more time playing together.
While United have not lost since their 7-0 thrashing at Liverpool on March 5th, Ten Hag said there would be no resting easy in their position with 10 league games still to play.
"It's game-by-game. We know what we're capable of and we see this team are still improving," he added.
Some good news for United is the imminent return of Brazilian midfielder Casemiro who completed his four-game suspension on Saturday.