It's entirely possible that playing at DM will have developed him into being a better CB, one that can read the game better and be able to support whoever might be playing that role in front of him if he is to end up as CB.He was pretty much the invisible man after his surprise appendicitis operation a year ago.
Personally really pleased to see him back against Red Star as a CB - and imo that role plays to his strengths of reading the game, solid defending and not needing to be athletic rather than a DM where in addition he needs to be able to pass the ball about - where in addition to doing the basics right he seemed to assume the captaincy/organisation of the defensive unit. And whilst Red Star weren't amazing o n the night getting a captain of the defence is really useful - Jan Vertonghen usually does that role but when Jan doesn't play their often seems to be a poorly organised defence. Worth remembering that Southgate made him captain of England a couple of times, the last time just over a year ago.
Early days for him back playing when 100% recovered after injury, so maybe we'll see him also back playing as a DM, but if not think he will be very useful for us this season playing as a CB. Worst case is that it means Alderweireld has to up his game to play.
Irrespective, IMO he's versatility will always be his strength. At the time of his sickness/injuries, he was no better or worse than Henderson. The major problem with either of these two players isn't the player but peoples interpretation of the role they occupy (talking DM here obviously).