The Brazilian missed over two months of action last season due to groin and calf injuries and is currently out injured, meaning Ait-Nouri's chance has come - a chance he has taken with both hands.
Whilst impressed with the France under-21 international's performance in Monday's 2-1 win over Everton, Lage remains keen on none of his players becoming comfortable.
"Marcal knows, when he comes, he needs to work hard because Rayan is doing well and he is doing well because he worked in the shadow during seven games because Marcal was doing well."
The Wanderers boss has previously brought up having competition inside his squad of players. With Jonny out since the start of April with another knee injury, competition at left-back is exactly what Lage wants.
"That's the thing we need every time to improve between our players," the 45-year-old said.
"We are happy with him. The quality he has, not just now, but also for the past, the way he plays in an offensive way.
"We talk with him and we work with him, if he's more consistent and if he wins more defensive duels, he can be a top player. Like Max (Kilman), what I want is, tomorrow again, another good performance."