The 22-year-old has enjoyed a resurgence under his manager in recent weeks, impressing in starts against CSKA Moscow and AFC Bournemouth.
But Mourinho caused some supporters to scratch their heads when he elected to throw on Marcos Rojo at left-back rather than Shaw during the latter stages at the Hawthorns.
And Cascarino, speaking soon after the game, insisted that Mourinho’s snubbing of Shaw shows that he isn’t taken seriously at Old Trafford any longer.
“He needs to leave Manchester United and restore his reputation as a top-flight left back.
“A footballer’s career is not long and he can’t waste his talent sitting on the bench – even if he will earn more money doing so.”
The hysteria surrounding Shaw’s exclusion on Sunday constitutes knee-jerk reactions at their most potent. On the surface, it doesn’t look great, sure. But if you delve just a few centimetres underneath and think about the whole thing for a second, it actually makes sense.
The fact that Shaw came off with cramp against Bournemouth shows that he is still some way off full match-fitness, which isn’t just about minutes on the pitch but also the period in between.
Mourinho clearly wants to use him – hence his placing ahead of Matteo Darmian and Daley Blind in the pecking order – but not so soon after giving him a heavy stint during the week. Seeing him pick up yet another injury after an encouraging couple of weeks would be the worst scenario possible.