His age:
The English international, who has 8 games for the national side and has made more than 80 Premier League appearances turns 22 after 6 months. He is still relatively at the start of his career and has already achieved some unthinkable things. He has failed to impress for a couple of seasons now but is still a player who is learning the trade and Mourinho would be harsh to not stick with him after just an year. The 21-year old is one of the best players in this position in the squad and he will only get better the more game time he gets. So United are better off by giving him more time to settle in, to try and reap the benefits of his quality later in the future.
Lack of replacements:
Although the Red Devils have the likes of Matteo Darmian and Daley Blind to play in that position this season, Luke Shaw remains the only player who fits the style of play at the club. Mourinho is not happy with either the Italian or the Dutchman and he will have to look into the transfer market to replace Luke Shaw if he makes the move. But there are hardly any viable options at the left-back spot which might leave Mourinho’s side a bit weak in this position. Shaw is a top class player who has what it takes to grow on to become a world class player and Manchester United are better off sticking with him , rather than trying to replace him with an overpriced defender.