The Old Trafford club won the Europa League with Mourinho as coach in 2017. He recently became the coach of Fenerbahce, who were knocked out of the Champions League in the third qualifying round.
United will also notably play at home to 2022 Europa League runners-up Rangers and go to Portugal to play FC Porto, another of Mourinho's old sides. In their other fixtures, they will play PAOK, Bodo/Glimt, Twente, Viktoria Plzen and FCSB.
Tottenham Hotspur, the other English representative in the competition, will also take on Rangers, in their case away from home.
Ange Postecoglou's side will also play at home to Roma and go to Istanbul to face Turkish champions Galatasaray.
Spurs will also play AZ Alkmaar, Elfsborg, and Qarabag at home and face trips to Rangers, Ferencvaros and Hoffenheim.
In other standout fixtures, Roma will host Athletic Club from Bilbao, Porto will face a trip to Lazio, while Ajax will play both Real Sociedad and Galatasaray.
About the competition
The format of the Europa League has changed for this season, along the same lines of the new-look Champions League.
The number of clubs in the competition proper is now 36, all of whom are placed into one league but split into four pots of seeds.
Each participant plays eight games, facing two clubs from each pot, one at home and one away.
The top eight at the end of the league phase - which runs until the end of January - will advance to the last 16, while the teams finishing from ninth to 24th will advance to a playoff round to decide the remaining last-16 spots.
The bottom 12 in the league phase will be eliminated, with no clubs dropping down into the Conference League ahead of the knockouts, and no teams parachuting down from the Champions League as in the past.
UEFA have said that dates for all fixtures will be announced on Saturday. The first group games will be played on September 25th and 26th, a week after the opening matchday of the Champions League.
This season's Europa League final will be played in Bilbao on May 21st next year. Atalanta won the trophy last season, beating Bayer Leverkusen in the final in Dublin.
