The Premier League champions failed with bids for several midfielders over the summer, with their pursuits of Thiago Alcantara and Cesc Fabregas ending in defeat.
And though United eventually clinched the signing of Marouane Fellaini from Everton for £27.5 million on deadline day, manager David Moyes has been linked with a move for Cabella.
"He is an intelligent boy who has made a good start to the season," Fernandez said.
"Now in the winter transfer window if Manchester United offers €18 million, as I see it, he must go.
"Even if you 'kill the team', you can't refuse."
Cabella has made a fine start to the campaign, scoring five goals and making five assists in Montpellier's opening eight games and Fernandez is hoping to see the 23-year-old in a more offensive role.
"We must see if he is able to assume the responsibilities," the 59-year-old added.
"If he is able to play as a number 10, these are the matches that will decide."