Manchester United must capitalise on ANOTHER Arsenal mistake in January transfer window

The opportunity to take a player off a rival will be one Mourinho relishes given his fondness for securing a psychological advantage.

And Arsenal could count the cost of letting another of their homegrown core depart, following the previous departures of Alex Oxlade-Chamberlain and Jack Wilshere.

Arsenal have previous for letting high-profile players run down their contracts, before and after Van Persie was allowed to do so.

And United benefitted from their failure to convince Alexis Sanchez to sign a new deal, even if the Chilean has only recently started to show glimpses of returning to top form.