Ashley Young becomes the sixth player to test positive at Inter Milan.
Inter Milan pull level on 54 points with Juventus but are ahead of the champions on goal difference thanks to a thrilling second half turnaround.
Inter Milan have brought in Christian Eriksen, Ashley Young and Victor Moses to keep the pressure on leaders Juventus.
Ashley Young spent the past eight-and-a-half seasons at Old Trafford since arriving from Aston Villa in 2011 for around 20 million pounds ($26 million).
Manager Jose Mourinho made four changes to the side against Fulham after their draw with Arsenal in midweek.
Young's first United goals since May 2016 were followed by an Anthony Martial strike as United produced a scintillating first half at Vicarage Road.
Zlatan Ibrahimovic's introduction as a second-half substitute proved crucial as Manchester United finally discovered a way to beat newly-promoted Brighton and Hove Albion.
Goals from Wayne Rooney, 18-year-old Marcus Rashford and substitute Ashley Young secured the three points, which was enough to lift Manchester United above Southampton into fifth place in the Premier League
Manchester United are a point behind second-placed Arsenal, who have played one match fewer, but Young is confident Louis van Gaal's third placed side can overtake the Gunners and finish as runners-up behind likely champions Chelsea.
Manchester United F.C. hit back through Ashley Young, Marouane Fellaini, Juan Mata and Chris Smalling to register a sixth successive league win after Sergio Aguero gave Manchester City F.C. an eighth-minute lead at a blustery Old Trafford.