There's a difference between being un-fit and UN-FIT. Most pro athletes carry niggling injuries. Kane probably has ankle issues on the regular since he has injured the same ankle 3 times. Ibrahimovic wears the tape because of back issues that are never going to go away 100%. But you can still play at a top level with the right preparation and precautionary measures, which is what tape like that is for. You never wear stuff like that for treatment, you wear it as a precautionary measure.
Victor suffered a hamstring injury in the 14-15 season so it would not surprise me at all if he still has issues or nigglers from that from time to time. And all it would take is one bad landing for something like that to flair up
Point being, if he wasn't fully fit to be starting and playing games. Pochettino wouldn't play him. Whether it's a confidence issue with Wanyama or not remains to be seen. Last year, he started the season very well, he was partnered by Dier and then Dembele and was part of a great season for us.
This year, he started the season poorly. Got injured, came back and has been even worse - if it is a problem with a hamstring injury, especially one he received previously and Pochettino feels he can't get the best out of Wanyama or get the Wanyama we got last season, then it would be best he moved on. Sandro is one that comes to mind. He was still fit enough to play games, but his injury seemed to have such an effect on him that he was never going to be the player he once was.