It seems reasonable to assume it's Covid related imo. We've seen it from time to time in other sports. Just like in society in general, most guys come back and are fine. But some struggle for a long time and it's usually down to respiratory issues. In the NFL, Myles Garrett was wrecking shop until he caught it halfway through the season. He came back after a couple of weeks and was nowhere near the same player, and never really got back to that level the rest of the season. He was complaining about being unable to catch his wind between plays. There are a couple of instances in the Premier League as well.
With Zib the drop in performance seems more significant than just not being 100% on his game. I'm speculating of course, but it seems the most likely explanation to me.