Ole Gunnar Solskjaer insists he will not run Marcus Rashford into the ground and risk burning him out.
Rashford has made 16 appearances this season, more than any other United player, and has started all nine of their Premier League games.
The England forward, who suffered a double stress fracture of the back last season, sustained a shoulder injury in United's 3-1 Champions League defeat to Paris St Germain in midweek.
Although Solskjaer is hopeful Rashford can recover to be included in United's squad for today's trip to West Ham, he said is not prepared to gamble with the 23-year-old's health and fitness.
“Of course, that's a medical issue - how serious is it?” said Solskajer.
“At the moment, Marcus is getting that tiredness towards the end, or that soreness towards the end, that stops him from completing the games.
"Hopefully it won't take too long to completely go away.
“Of course you think about how you manage these players throughout the whole season, anyway.
“Marcus, he'll run through a brick wall for you, and that's the type of lad he is.
“We've just got to keep our fingers crossed it doesn't get worse, because at the moment it's still manageable.”
Solskjaer comes up against David Moyes on Saturday, who lasted less than a season at United after succeeding Sir Alex Ferguson.
Since Moyes, United have continued that short-term approach, with Louis van Gaal and Jose Mourinho preceding Solskjaer, who will mark two years in the job later in a fortnight.
In contrast to his recent predecessors, Solskajer appears to have the long-term backing of the United hierarchy and said: “From my experience, from my discussions with the club, we think long-term.
“Of course, you've got to look at the short-term results as well, but my vision is long-term and my discussions with the club have always been long-term.
“But you can't not think about the next six games.
"You've got to think about the next game and the next six, but also plan and think for the bigger picture.”
Solskjaer acknowledged the unique pressure of the United job but said: “You wouldn't want it any different.
“We're lucky that we have so many fans, and you just have to withstand or enjoy whatever pressures that will give you.”
The game at the London Stadium will be the first in the Premier League since fans were readmitted.
Solskjaer said: "It'll be great playing in front of fans and we're looking forward to it.
"We won't have our fans there, but we'll have to use the energy and thrive off the home fans and try to make them quiet and edgy."