The Ronaldo performing good last season is extremely exaggerated and actually a bit of a myth.
He played 30 league games last season and got 18 goals and 3 assists, but his 18 goals came in 12 games, he didn't score in 18 league games, which for a player that only offered goals and not much else in his all round game is actually not great.
18 games where the team was hampered by only playing for him yet him not contributing is part of what cost us last season.
United finished on 58 points last season, and Ronaldo's 18 goals and 3 assists only gained the team 15 points. To suggest a different striker couldn't help the team gain more than 15 points is questionable.
Ronaldo scored 10 of his 18 goals in 4 games. So in the remaining 26 games he played he only scored 8 goals.
He had a period where he only scored 1 goal in 8 games. He also had a period where he only scored 2 goals in 10 games.