I don't know what you've been watching. Smalling had a few decent performances but it wasn't a "very high standard". To say Smalling played at a high standard is comparing him to the likes of Boateng, Godin, Silva, Ramos, etc. He was never on that level. Not even close.
Rojo I think, can be. He's pacey, aggressive, gets stuck in, doesn't hesitate. His marking is good, he's great on the ball and he always looks "up for it"...
Smalling on the other hand is like bamby on ice at times. Doesn't look like he can control a football and mostly relies on his physical presence to make up for his lack of everything else. He's basically Fellaini but a bit quicker. He's prone to shirt pulling and letting in headed goals too. Whenever I see him play in a big match, I always think he's a liability and most times, I'm right. I go back to the City game a few years ago when he got a red card and pretty muchc cost us the game... and then there was the FA Cup final last season where he stupidly got sent off and could have cost us a trophy. And then of course, getting destroyed by Chelsea this season which I think signalled the end of his United career. He's everything people like to think Rojo is.