Ole Gunnar Solskjaer has been warned he is under "severe pressure" as he prepares for a make-or-break match in his Man Utd tenure.
The Norwegian was again left facing questions over his team after United slumped to a disappointing 2-1 defeat to Turkish outfit Istanbul Basaksehir on Wednesday.
United have been criticised particularly for the manner of Demba Ba's opening goal, which laid bare the problems Solskjaer is struggling to deal with.
The defeat is United's second in a number of days after they were beaten 1-0 by rivals Arsenal, and exactly a month after their horror show against Tottenham.
To make matters worse for Solskjaer, ex-Spurs boss Mauricio Pochettino appeared on Sky Sports on Monday talking up his potential return to management.
Now pundit Paul Merson feels United's clash at the weekend will be Solskjaer's biggest of his managerial career.
"I think Solskjaer will be under severe pressure come tomorrow. Let's be honest, this is Manchester United," Merson told Sky.
"This is, for me, the biggest club in world football. Where they are in the league is not acceptable.
"It's not like it's two or three games gone. It's not just that but the performances have been poor.
"The way they capitulated against Tottenham. Arsenal weren't great the other day they just done a job, they [United] looked lost.
Should United part company with Solskjaer? Have your say below.
"And then they've gone to, let's be honest they're [Istanbul] not a top European team and they've been beaten."
He added: "I think it will be Solskjaer's - if he gets to Saturday - his biggest game in management.
"Pochettino was on Sky on Monday. People want to be seen. Managers come on Sky and people go, 'oh, I forgot Pochettino was out of a job'.
"I'm not saying Man Utd do but people do and fans do and it gets going, and I think Saturday if he's still in a job it will be the biggest game of his managerial career so far."
