Brown played 20 times, notching three goals, for the bottom-of-the-table Millers, who are threatening to “get in his ear”.
He already has four in seven outings for Town, his latest strike the opener in Saturday’s 2-1 win at Queens Park Rangers.
Other than two Premier League appearances from the bench (one for previous club West Bromwich Albion, one for Chelsea) the 20-year-old’s only other taste of senior football came while on loan to Dutch club Vitesse Arnhem last season.
“Izzy has all the offensive skills, just small spaces to improve in,” said Town boss Wagner, whose third-placed side are seeking a fifth straight league win and 10th in 12.
“We have tried to help him with defending and also to bring him in a good position for the offence.
“I think we have also helped him with how to play as a man, not a youth footballer.
“And we have worked to reach the fitness levels he needs for our high-intensity game.”
Rotherham first-team coach Matt Hamshaw said: “Izzy’s a good player who can play at any standard he wants if he puts his mind to it.
“But we might be in his ear a bit!
“All credit to him but we won’t be wishing him all the best until after the game.”
Hamshaw said of Town: “They’ve got good players and I wouldn’t be surprised if they got automatic (promotion).”