The Portugal international returned to Old Trafford from Juventus in the latter stages of the transfer window and has scored five times in seven appearances for the club in his second spell.
The Red Devils have lost two of their last three Premier League games, though, and Ronaldo has found it difficult to contribute in the recent clashes with Aston Villa, Everton and Leicester City.
Merson is of the opinion that Solskjaer was not fully behind the club's decision to re-sign the former Sporting Lisbon youngster on a two-year contract.
"Manchester United have brought all these players in and you're left scratching your head as to where Solskjaer fits them all in," Merson told Sky Sports News.
"To this day I still don't get the Ronaldo signing. Hand on heart, would Manchester United have gone and got him if Manchester City weren't linked? Was Ronaldo even going to City, or was he just playing them to get back to United?
"I'm not 100% sure Solskjaer wanted Ronaldo there, I may be wrong, but his decision to leave him out against Everton told me all I needed to know.
"I think he saw it as a good opportunity to beat a team without Ronaldo and then carry on from there, but they didn't, and everybody went mad. But would they have beaten Everton with Ronaldo in the team?"
Man United will be looking to return to winning ways when they take on Atalanta BC in the Champions League on Wednesday night.