Barry is on-loan at Hull from Aston Villa.
"He did really well, especially in the first half," Selles told Hull Live after Hull's draw with Norwich. "We spoke about it in the press conference about how we can build them (up to fitness), and that kind of player like Louie with no experience in the Championship and coming back from the injury before he went back to Aston Villa.
"After 45 to 50 minutes of his performance, his physical levels dropped, and we cannot allow the team to have a player drop those physical levels, so he will be fine. He will continue evolving, and I think he did a good performance.
"He's always a threat when he has the ball, and hopefully, that performance will transform into assists and goals, and he will be as good as any other in the league.
"I expected it to take relative time, so I didn't expect him to arrive and immediately smash the league. I know he's a really good player.
"There's going to be a moment when he's going to get that movement right, and he's going to get that finishing right, and that is going to unlock something.
"It always goes like that, but to arrive at that point, you sometimes need to suffer a little bit, and I think he will be fine, and he will be important for us."