The Red Devils were preparing to face West Ham on Sunday at the London Stadium with the team doing their usual warm-ups.
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That included some of the players taking shots at goal, with Weghorst among the cohort trying to test David de Gea in the net.
But the on loan striker's effort was wayward, to say the least, flying over the bar and into the crowd, where people were starting to find their seats.
One supporter was not paying attention as Weghorst's shot glided into the stand with some curl on it, smacking the fan on the side of the head.
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In truth, the fault does not lie with the fan considering Weghorst was mere yards away from goal before booting one into the London Stadium stands.
The Netherlands international started the contest against West Ham but his wild shot was a sign of things to come after a miserable performance from United.
The Red Devils succumbed to a 1-0 loss to the Hammers after a horrible mistake from De Gea that saw Said Benrahma's shot squirm underneath him.
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On the other side of the pitch Weghorst had little impact and was brought off for Anthony Martial in the second-half.
But it made no difference as United went on to lose with just a point that separates them from fifth-placed Liverpool in the Premier League table.
Nevertheless, Erik ten Hag will still believe he has an advantage as United have a game in hand over their old rivals.
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But their form has taken a hit having won just once in their last four Premier League matches.
And whether Weghorst remains past the current season looks unlikely after joining on loan from Burnley in January.
In 27 appearances for the Red Devils, the 30-year-old has scored just twice and has failed to net in the Premier League at all.
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Ten Hag has made signing a striker a priority in the transfer window with Harry Kane and Victor Osimhen top targets.
That would seemingly leave Weghorst on the sidelines, though Ten Hag could opt to sign him permanently as a squad option.