Speaking to BBC's 5 Live Breakfast, Gary Neville said: "Ole Gunnar Solskjaer dealt with Paul Pogba well over the last 12 months. He's dealt with big personalities. He seems to have the right temperament for that.
"There is no doubt Cristiano Ronaldo is a make or break signing for Ole because, with Ronaldo in the squad, he has to win. You can't have any excuses. You bring one of the best players of all time into your club.
"With the Jadon Sancho signing and the Raphael Varane signing at centre-back, he has now got to deliver this season. He has not won a trophy yet with the club, and he knows now is the time to deliver."
Manchester United reached their first final under Solskjaer last season, losing to Villarreal on penalties in the Europa League while finishing second in the Premier League.
The Red Devils have now gone a total of four years without winning a single trophy, their worst drought since the end of Ron Atkinson's tenure and the start of Alex Ferguson's one.
