The 23-year-old midfielder has enjoyed a productive opening to the 2013/14 campaign, scoring five goals and laying on four assists.
United spent much of the summer chasing creative targets and are reported to have now turned their attention to Cabella.
Montpellier are aware of the interest and concede that they would have little option but to sanction a sale if their valuation is met.
Coach Jean Fernandez told Reuters: "He's an intelligent boy who has made a good start to the season.
"Now in the winter transfer window if Manchester United offers €18m, as I see it, he must go.
"Even if you 'kill the team', you can't refuse."
Cabella has been booed by certain sections of the Montpellier support over recent weeks, but club chairman Louis Nicollin insists a player who was strongly linked with Marseille over the summer remains fully committed to the cause.
He said: "There are a lot of fools among the fans. Those who boo are not real fans.
"The kid always gives his best. He gives a lot, so booing him is unfair.
"Marseille's interest? He learnt everything here and is particularly grateful. We don't change our mind every morning. We said we won't sell, so we didn't sell him."