Greenwood hasn't been allowed to train at Carrington ever since he was suspended by the club as charges of attempted rape and assault were levied against him 18 months ago.
Gubba, who runs the 'Man Utd The Religion' YouTube channel, caught a glimpse of the United striker training at a public park and filmed him going through a few drills in a session monitored by his father, Andrew.
Members of Greenwood's entourage weren't happy at all, as Gubba alleges that they went as far as threatening him with the destruction of his equipment.
The 21-year-old is still under contract at United despite all that has gone on and, after all charges against him were dropped in February, there is a decision to be made regarding whether he should be re-integrated into the United squad.
Any such decision is expected to be made in the upcoming days, with Ten Hag wanting the situation cleared up before the start of the Premier League campaign.
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Gubba claims that one of Greenwood's associated said: "If you weren't surrounded by witnesses we would rip that camera out of your hand and destroy your equipment."
Greenwood's future is still well up for debate. Should United decide to keep him around, it's highly unlikely he'll be featuring in any matchday squads for some time as he'll have to build his fitness back up to the required level.