Radamel Falcao has already left the club following a disappointing loan spell while James Wilson needs more time to develop.
Wayne Rooney would be left as the only viable option so there is no doubt that van Gaal would be in for a new forward if van Persie was to move on.
We asked our Twitter followers for their thoughts on who Man United should be looking to bring in:
@PeoplesPerson_ Muller, look at what he's achieved already, still only 25!
— David Balfour (@DavidBalfou) July 2, 2015
@PeoplesPerson_ test PSG's mettle for Zlatan
— Adrian (@teandbusquets) July 2, 2015
@peoplesperson_ lacazette or Benteke
— Rajat (@ScholesyLad) July 2, 2015
@PeoplesPerson_ Benzema ideally. Wilson still needs time for sure. Müller isn't usually a centre forward.
— Elliot Saunders (@saunders_MUFC) July 2, 2015
@PeoplesPerson_ Muller or Benzema – but both aren't leaving their clubs any time soon. Don't rate Cavani so Wilson! Back the youngster
— Unitedsway (@Unitedsway) July 2, 2015
@PeoplesPerson_ would love to see Muller play for us, he's one of my fav non Utd players, Wilson needs a season loan imo
— 48:13 (@johnmufc20times) July 2, 2015
— Anders (@Sly_MUFC) July 2, 2015
Of course, world-class forwards are hard to come by at the best of times and it will be difficult to tempt the likes of Thomas Muller and Karim Benzema to Old Trafford but United should always be targeting the best players.
Whoever United decide to sign, it is important that they do their homework on the player.
Falcao was desperately poor at times last season and the loan has to go down as a huge failure. The club cannot afford a repeat if they are to be successful on all fronts next term.