United don't struggle with physical beasts they have Phil Jones, Anderson, Cleverly, Carrick who can all hold their own on that front. They suffer from complacency. They look prepared to absorb pressure but then shit the bed and come out saved by the skin of their scummy yellow teeth. Then they kick into gear, string together a few incisive passes and score on the next run of play.
United fans who think they get bullied by teams who just push and shove are IMO, trying to find an excuse for complacency and poor awareness because in an ideal world, physical bullying is the "wrong" way to play football. They got outplayed by their piss-easy CL group last year and it was because they were caught with their pants down with their little red peckers up their own asses just like City did (and this year as well), not because the opponents bullied them into the grass.
I don't think Ronaldo will be a problem more than Özil. Özil finds space everywhere he goes and intelligent attackers with space around them in the final third are what really put United on their asses, moreso than pace, strength or other specific attributes. Ronaldo may score one or two in this tie but I think Özil will be the one to get credit in the end. That's just my guess though.