We can sometimes focus so much on a player's weaknesses that we ignore that they're actually pretty good. Pogba's defensive awareness, Rashford's technique, Martial's movement, Maguire's pace, AWB's attacking, etc have all been used as reasons why they're not good enough.
The thing is, if we get so hung up on that, we start to think they're bad players instead of realizing they simply need their strengths magnified and weaknesses hidden. For instance, Martial didn't run in behind that much on the left, causing accusations of poor movement and mentality. At the same time Rashford was accused of poor technique and hold up play for a striker. Instead of binning them off, Ole switched their positions, and suddenly they're both firing and have good synergy. You can see the same with Pogba as when he played in a 3 as the more attacking mid as opposed to a 2 with more defensive duties, the performances were much better.
I think if every one of our players is put in a position to succeed, they'll show they're actually quite good, and Smalling is no exception. His technique isn't good, but he's been part of one of the best defences in the league several times now. If he's put in a situation where his superior defensive qualities are important and his lack of ball handling isn't, he'll be viewed as a top CB. It's probably not going to be at Utd though since Ole wants both CB's to be good at ball handling, even if it's at the expense of defensive ability. And that's fine if that's what the boss prefers, but it doesn't necessarily make Smalling a bad player.
And to be honest, I think Smalling and Maguire theoretically fit together and hide each others' weaknesses much better than Lindelof and Maguire, but at this point it's probably too late to go down that road.