The England international was poised to leave Old Trafford before the transfer window closes but has been sent for scans on a knee injury.
The striker is available for around 19 million euros but was sent for scans this week after complaining of pain in his knee.
Louis van Gaal's side has moved a step closer to signing Angel di Maria, with it being revealed Saturday that the Argentine wants a move to Old Trafford, and thus Welbeck's United future has been placed in further doubt.
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The England international's preferred position is as a central striker but he is behind Wayne Rooney and Robin van Persie in the pecking order and could now see his path to the starting XI further blocked by Real Madrid star Di Maria.
Van Gaal has perservered with Ashley Young in a wing back position of a 3-5-2 formation during preseason and in United's opening day defeat to Swansea City, although a similar role for Welbeck would appear unlikely.
United is now awaiting the results of Welbeck's scan but any underlying issue would undoubtedly put potential bidders on hold.