Marouane Fellaini, Jesse Lingard and Antonio Valencia’s goals had earned Jose Mourinho's side three points, with Rudy Gestede’s late strike briefly offering the hosts hope of a comeback.
Shortly after pulling the score back to 2-1 in the 77th minute, Gestede clashed with Bailly, holding the defender around the neck. Both players had to be pulled apart by their team-mates.
After the final whistle, tempers flared again, with television cameras showing several United players congregated in the mouth of tunnel. Bailly and Young look to move down the tunnel, only to be prevented from doing so by Middlesbrough stewards, officials and their own teammates.
When questioned on what had happened at full-time, United’s opening goalscorer Fellaini claimed “nothing” had happened.
Steve Agnew, the Middlesbrough caretaker manager, also denied all knowledge of the incident, telling BT Sport: “I didn't see that. I was inside quickly, so I didn't see that.”
United defender Phil Jones believed it was little more than an expression of frustration from the hosts, who remain stuck in the bottom two of the Premier League following the defeat.
“I think it was a bit of frustration creeping in on their behalf,” he said. “It is a difficult situation, I was in it with Blackburn so I know how they feel. When emotions are high these things happen but it was something and nothing.”
Gestede also denied that he had bitten Bailly during their initial clash. “We had a few words together, it was nothing major,” the Middlesbrough striker said. “He gave me a hug and a kiss, that's it. I didn't feel anything. Ben [Gibson, Gestede's team-mate] thought he bit me, so that's why he reacted like he did. But I told him now we were just talking, there was no problem.”
Additional reporting by PA
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