Fernandez has admitted that Montpellier would be unable to turn down an offer in the region of £15m for his prized asset, despite Cabella being the creative hub of his improving side.
He told Reuters: "He is an intelligent boy who is enjoying a good season.
"Now, if in the winter transfer window Manchester United offer €18m (£15m) as I have read, he must go.
"Although that would kill the team, one cannot refuse an offer like that."
The 23-year-old has made a strong start to the campaign, scoring five goals in eight games including a brace against Lyon last time out.