Yeah, I get what you’re saying, but remember how skinny John Stones was when he first came in. He didn’t look built for the league at all at that point. Over time he filled out, got stronger, and learned how to handle himself physically.
That’s pretty normal for young players. As they get older, their bodies mature, they add muscle, and they just become more street wise in duels and contact. You don’t usually see the finished article at 20.
I get the concern about the physical step up, but Vitor Reis isn’t coming from a soft environment. The Brazilian top flight is a very physical league, matches played on tough pitches against aggressive and relentless strikers like Gabriel Jesus and Igor Jesus who don't stop hassling you all game. It’s proper battle hardened football.
Reis broke into a title contending Palmeiras side at just 18 and became a regular - which doesn’t happen unless you can handle yourself physically. He’s already shown strong physicality, aerial ability and strength in duels despite still developing - as per his statistics. He’s very tall at 6'2, wins plenty in the air, and doesn’t seem to shy away from contact. I know it was only Plymouth in the cup, but we saw him up against a tall, strong and powerful striker and he was more than capable physically.
So while there will always be an adjustment to Europe, he’s far from fragile even at his age.