Waiting game: Bryan Mbeumo
The forward is a key target this summer for United, who have seen two bids rejected by Brentford for his services.
The Bees want more than the £62.5million United paid for Matheus Cunha earlier in the window, with the second and more recent bid totalling just shy of that figure.
Standard Sport understands both Brentford and Mbeumo are growing frustrated at the slow progress over the potential deal, with United yet to table a third bid more than two weeks after their last.
Tottenham and Nottingham Forest's interest in Yoane Wissa further complicates matters, with Brentford determined not to lose both star forwards in the same summer.
Spurs have been quoted £50million for the striker, a price which the club and understood to have baulked at with Wissa turning 29 years old in September. Forest have seen a bid worth around £20m rejected this week, having reignited their interest from January.
Mbeumo has made it clear he only wants to join United, having supported the club since childhood, and has sent a timely reminder of his wish with his social media activity.
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Eagle-eyed fans have spotted that Mbeumo liked Leny Yoro's Instagram post about returning to pre-season training at United's Carrington training base.
Yoro posted an image of his training with the caption "ReUnited", which Mbeumo liked despite having no prior connection with Yoro. Bruno Fernandes and Diogo Dalot also sent their approval.