Good squad player? Man Utd played 25 games under Ole, Mata only provided 2 goals and 0 assists (1 vs Reading at January and 1 vs Chelsea at April, which is three months later), 11 games not starting for any mins, 10 games for starting line, but within those 10 games, 4 games being substituted before 65 mins. The only good games that he played under Ole is vs Reading, vs Fulham and vs Chelsea first half, so basically he had no performance for 22.5 games out of 25. Is it what we called a good squad player?
Creative player? As i mentioned above, 0 assist under Ole takes charge, 2 assists for the Premier League this season, same as Lingard, who scored more goals than Mata for the last 2 seasons, but was criticized by many Man Utd fans.
Good leadership? Then how come Martial, Pogba didn't learn from his attitude, Rashford became more and more arrogant? Because they all think Mata's performance and position in the team is way below them, and cannot convince them to listen him.
The reason he is still at the team is only based on his past reputation on and off the pitch. If Ole plays 4-3-3 as our main formation next season, we don't even need to buy a player to replace him, because there is no pure no.10 position for Mata, and there is no conflict between buying a right winger and giving up a no.10.
ST : Rashford ; maybe M. Dembele ; Greenwood
LW : Martial ; maybe D. James
RW : a new right winger ; Lingard
And i already not mentioned about his high salary, and block the way for Angel Gomes, which has 2 years contract remaining, and is linked to PSG. I can't think of any reasons why we still need to keep him if we really want to rebuild our team to complete with Man City and Liverpool.