Smalling is near 30 and had one very good season under lvg and a whole host of crap ones mate, he's not now and never particularly was a pretty good CB, lindelof may end up being a success, but so far two years into his stint at united he's been varying between decent and shockingly bad, Luke Shaw has done that now for four years though and at RB they have who exactly?
There's a reason mourinho melted down at not getting in a defender or two, because the ones he has aren't fit for purpose.
Soto answer your earlier point, yeah some of our players would be better than their united counterparts unlike you seem to believe.