I am not sure where you get your statistics but I dont believe ten Hag is statistically one of our worst performing managers in the PL era.
Jose has the best statistics among Utd managers since Fergie:
Win Percentage: 58.3%
League Positions: 6th, 2nd
Trophies: 2 (Europa League, League Cup)
ten Hag:
Win Percentage: 56.7%
League Positions: 3rd, 8th
Trophies: 2 (League Cup, FA Cup)
van Gaal:
Win Percentage: 52.4%
League Positions: 4th, 5th
Trophies: 1 (FA Cup)
Ole:
Win Percentage: 54.2%
League Positions: 6th, 3rd, 2nd
Trophies: 0
Moyes:
Win Percentage: 52.9%
League Position: 7th
Trophies: 0
I would say I will give him until December to assess his performance. I am not just looking at the results but also at the overall performance and progress. Yes, I expect a top 4 finish this season or 5th place and a Europa League win.