Actually in this case there is no reason for Ronaldo to leave especially for Roma.
Even though it's the worst possibility now that Ronaldo has moved from MU, it's clear that his choice is not Roma.
The reason is quite clear, he wants to gain experience and doesn't want to be separated from the Champions League and Roma certainly can't do that so why would he leave if he still can't be in the Champions League.
If Ronaldo leaves for Roma (although chances are slim ) he might not be too worried about playing in the champions league. He might be yarning for a different adventure in the Seria A. So far Ronaldo has achieved a lot that one more season of not lifting the CL trophy might not mean much to him. He could have something bigger he is hoping to achieve with José Mourinho. Perhaps if Ronaldo remains in united he won't still play in the champions league
It's true, that when Cristiano Ronaldo left was probably one of the reasons he wanted to play in the Champions League. But in my opinion, it would be even more extraordinary if Ronaldo stayed with Manchester United and helped Manchester United to get back into the Champions League. It's quite difficult indeed, but I want to see it actually happen.