The forward is said to be in line to become the club's top earner with a new deal worth £375,000-per-week.
After the best year of his career so far, Rashford is keen to kick on and become the main man at Old Trafford after scoring 30 goals in all competitions last term.
However, according to former Chelsea striker Cascarino, the 25-year-old is yet to prove himself as worthy of earning such a deal.
Speaking on Weekend Sports Breakfast, Cascarino said: "Look, he's a very talented lad. He went through a number of injuries and had problems off the pitch which he seems to have resolved but now he's set the standard of what he's capable of.
"If you go three or four years down the line and we're talking about Marcus Rashford, do we expect him to get near the 200 club? At the moment he's got 128 goals and 76 Premier League goals.
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"Can he get to 150 Premier League goals in the next three or four years? I'm not sure he's capable of it. I haven't seen enough evidence. He hasn't achieved 20 once in the Premier League.
"There will be people who say that he doesn't play down the middle as he plays off the left, but Ronaldo was off the left and in the middle and his record of how quickly he got to 100 goals was pretty alarming.
"Rashford had a fantastic season this year. The season that's just finished, 30 goals and 17 Premier League goals, but do you think you'd want to pay £375,000-a-week for someone who has had their best season at 25 years old and played very well and looks like a different player this year.
"He looks way more clinical than he has done, but I still am not totally convinced about Marcus's ability to lead the line at United for the next two or three years.
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"He has to have the season that he had this year and that has to be the minimum."