Shaw made his first league start of the season in the 1-0 win over Bournemouth earlier this month and was praised for his performance by Jose Mourinho. But just when the 22-year-old may have hoped to get a regular spot in the team, he was left on the bench in United's 2-1 win over West Brom.
The former Southampton man, earning £130,000 per-week at Old Trafford, has been a bit-part figure for the 20-time English champions in the 2017/18 season. The left-back missed a major part of his time at Old Trafford with a broken leg and has struggled to make his place in the team since returning from recovery.
Ashley Young has been recently preferred at left-back, while Matteo Darmian and Daley Blind were also ahead of Shaw in the pecking order. The England international has been linked with a move away from the club, but is said to want to stay and fight for his place, especially since Mourinho changed his stance in recent weeks.
United are said to be open to allowing the defender to leave the club in January, but will be demanding a fee of around £20m ($26.6m) from interested clubs. Newcastle United are said to be keen to take Shaw to St James' Park, but are looking at a loan deal rather than a permanent transfer.
Shaw could return to the United starting line-up on Wednesday when United take on Bristol City with Mourinho likely to shuffle his team ahead of the busy festive period.
"Luke Shaw remained on the bench yesterday and watched a winger - Ashley Young - and then a centre back - Marcos Rojo - play in his position," Cascarino told The Times, as quoted by the Express. "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."