McGrath insists United cannot afford to lose the silky-skilled starlet, who signed a new five-year deal with the club in October.
He told the Daily Star Sunday: "If United cannot manage to hold on to an emerging player such as Januzaj they can find themselves in real trouble.
"He's only 19 and it is clear he has all the talent in the world.
"You do not want to lose that sort of player."
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