The 23-year-old has made a fine start to the season, scoring five goals and laying on four assists in his first eight games - which could result in a January move to Old Trafford
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 they eventually clinched the signing of Marouane Fellaini from Everton for £27.5m on transfer deadline day, manager David Moyes is still in the market for midfield reinforcements - with Cabella a potential target.
"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 €18m, 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 the Stade le Mosson club'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."