The 18-year-old rose to stardom under Louis van Gaal last season with a series of sensational performances at No.9.
Mourinho, on the other hand, has chosen to deploy the youngster on the wing this season, with Zlatan Ibrahimovic leading the attack instead.
Cascarino, speaking on TalkSPORT, slammed the Portuguese for not giving Rashford a chance to impress up-front.
“I just thought – start of the season, you bring Zlatan in, whatever you do get the kid in alongside him. Just give him a chance to show how good he can be.
“I’ve seen Rashford deteriorate quite a lot this year, and there is no other reason because he has been taken out of the firing line.
“Last year he was absolutely sensational at the end of the season for United and now I am looking at a young lad that has been given a new deal, but been taken out of the firing line, not playing much football now.”
Rashford has featured in 14 of the 18 games United have played this season, so Cascarino’s claim that the Englishman is “not playing much football now” is woefully off the mark.
Here’s another questionable assertion: “I’ve seen Rashford deteriorate quite a lot this year.”
Now, I presume he means this season, as opposed to his meteoric debut campaign. Even so: four goals, a ‘Rising Star of the Year’ award, second place behind Renato Sanches in this year’s ‘Golden Boy’ award, a debut hat-trick for the England Under 21s, and a series of rip-roaring performances on the flank.
If this is what ‘deterioration’ is, then long may it continue.