The Foreign Office confirmed it was in contact with French authorities after the incident on Wednesday night, with thousands of supporters having travelled for the decisive second leg of the last-16 knockout tie.
“We are in touch with the French authorities following an incident involving a British man in Paris on 6 March and stand ready to offer assistance if required,” said a spokesman.
Sky Sports News reported that the man, 44, was taking a taxi back to the city centre from the Parc des Princes stadium before being attacked, and was taken to a local hospital for emergency surgery.
No arrests have been made.
United won 3-1 on the night after losing the first leg at Old Trafford 2-0, which ensured they progressed to the quarter-finals thanks to the away goals rule.
The Red Devils will find out who they face next when the draw takes place next Friday.
