What's actually wrong with him fitness-wise? Is he just delicate? Because it seems he can take a tackle.
I'm just making an educated guess, but my opinion is that he's very slight, he's been kicked A LOT in his career, then added to that the extra physical intensity of the Prem (that a lot of great players struggle with), and then a Covid season with games every 3/4 days most of the season, plus travel to play in games in South America is just an insane workload.
I've been pleasantly surprised that we've got 22 Prem games out of 31 out of him, tbh. If we can get 25-30 games a season out of him, it more than justifies the transfer, for the next level quality he brings.