The Brazilian midfielder has battled for a start in recent games.
O'Neil said, “He’s working very hard, but I think the central midfield area is one of our strongest areas. I was very impressed with him in his first few appearances, just the job against Manchester City maybe didn't suit him quite as well, and then he ended up losing his space at half time, and from that moment we've managed to be unbeaten.
“From putting a real good display in that game against Manchester City and then went to Brighton did well, played against Crystal Palace, played against Southampton, so it's just one of those moments for him. But he still works hard, he still shows his quality and he's still adapting to the difference.
“If you talk to him about the difference in football and the speed of the football and how it's played, it's a completely different game to what he's been used to playing, so he's still adapting to that. But he's going to be big for us, and we'll use him. Sooner rather than later, he'll have a massive impact on the team.”