People are so obsessed with this idea of ball playing defenders that they completely overlook the fact most importantly the defenders at any top team need to be able to be excellent defenders first and foremost. Smalling isn't coveted by top teams because while he is an excellent defender, they are looking to find a defender that has both in them - of that there are very very few top ones available.
For United though right now, OGS said it himself, is that what we need is that foundation in the back knowing we are not going to concede, from open play and from set pieces. Smalling to that end is still our best defender and with Lindelof's improvement, think will be a good bedrock for the rest of the season.
The other thing about Smalling is if paying attention to the pattern of play, there is nothing magical about what our CBs have been doing the last 4 games in terms of ball playing. They are not playing defense splitting passes or long cross field balls or chipped balls. All they are doing is moving the ball quickly to the likes Matic, Pogba and Herrera. Occasionally they've stepped out of defense with the ball at their feet and if anything, this is something Smalling is very capable of as well.
Smalling is also a threat in the opponents box. Not that we've had a lot of opportunities earlier in the season because we'd hardly win corners or free kicks in dangerous positions because of how little we were in the attacking 3rd.
Personally am looking forward to him getting back in this team. He has been our best defender and will just add a lot more confidence to that backline, which in turn will let us attack even more.