The 18-year-old, who was born in Brussels, is undecided on the issue of which country he will represent at international level, as he is also eligible to play for Albania, Serbia, Turkey and Croatia.
England have also expressed an interest in the midfielder, and he would qualify to play for the Three Lions in 2016 under FIFA's five-year residency rule.
Januzaj has turned down calls from the Belgium squad, and reports suggest that he wants assurances that he will be first choice, but Wilmots stated that he would not kowtow to demands.
"It is impossible to negotiate with me," the 44-year-old told RTBF. "The boy was born in Belgium, should he say that he wants to play for Belgium - from there, there is no haggling. I'm not going to play that game.
"The kid is available? He wants to play? Then perfect, OK, show me. If you're at the top level in Manchester or elsewhere, show me your skills and if I need you I will call. I make no promises, to Januzaj or any person."
That has prompted a negative response from Abedin Januzaj, who further hinted that his son is not decided on his international future.
"This is shameless," he is quoted as saying by the Belgian media. "We never reacted on selections, phone calls or letters from the Belgian FA. They are requesting his selection, not us.
"Wilmots's remarks won't make it easier now. We are angry about these comments."
Januzaj joined United from Anderlecht in 2011.