That is the opinion of Sam Allardyce, who was appearing on talkSPORT.
The United youngster reached his 150th game for the Red Devils at the weekend and crowned his achievement with a goal in a 2-1 win over Brighton.
It was his fifth in seven games under Ole Gunnar Solskjaer after seeing his progress stunt during Jose Mourinho’s regime.
The Norwegian has made Rashford his main man up front, dropping the likes of Romelu Lukaku and Alexis Sanchez.
As well as reaching a century and a half at Old Trafford, the 21-year-old also already has 31 caps for England.
But former Three Lions boss Allardyce reckons the youngster will burn out by the time he reaches 30 years old.
He said on Alan Brazil’s Sport Breakfast this morning: “It will be very difficult for him to still be playing at 30, because of the demand physically and mentally and the fatigue he will suffer.
“He’ll have three weeks a year off. That’s it. There’s no pre-season any more.
“They go playing straight away in these tournaments, he’ll play all over the world and he’ll play for his country.
“Even if he doesn’t get any major injures, by 25 he’ll have niggles just from wear and tear.
“The knees and ankles, the hips, his back will start being a problem as he goes on.
“It wont stop him playing but I think later on in life it will bring his career to an end quicker.
“If you’re looking at the Premier League now, there are fewer players over 30 playing now than ever.”
And United fans were not happy with the observations of the former Crystal Palace, Bolton and Everton manager.
One fan said: “Absolute fossil you are Sam.”
Another said: “What an absolute dinosaur. Does he not realise how fit players are nowadays and well looked after?
“Nutritionists, Sports Scientists, Strength and Conditioning coaches to name a few. No wonder he can’t get a job anywhere. Go back to bed Sam.”