The 23-year-old was a regular starter in pre-season but lost his place to Chris Smalling for the visit of Tottenham Hotspur, with his fellow England international producing an outstanding performance alongside Daley Blind.
It has now been revealed that Jones has mild thrombosis and the blood clotting has forced him to be sidelined for the opening of the campaign. Consequently, the centre-back is expected to miss the two Champions League play-off ties with Club Brugge.
"It was very peculiar because he had tightness in his calf the whole week and he kept training," commented Louis van Gaal. "But we have a very good doctor [Steve McNally], I think, because this doctor discovered a minor thrombosis. We don't take the risk so we went to a specialist at the hospital for a second opinion.
"The specialist said we have a new medical treatment and we have to keep him out for three weeks and watch him.