If you like fullbacks that can defend first, it's hardly surprising that a centreback playing there appeals to you. I distinctly remember Armand 'worst player in the world' Traore going past Smalling like he wasn't even there at least on a few occasions in the game at Old Trafford. He's decent, but nothing special. Just like Cleverly, just like Anderson, and just like Welbeck.
Put those 4 in our team that day and they would have probably have fared equally as bad or worse than the players we had in those positions that day.
United could have had Fergie at centre midfield that day and still won. Two teams in completely different places mentally, and that's not trying to "rationalise" anything, it's just fact.