Sociedad were given a chance to begin an unlikely comeback when a clumsy challenge from Daniel James saw them awarded a penalty in the 13th minute. But captain Mikel Oyarzabal contrived to send the spot-kick high and wide of the goal.
United then had chances to add to their aggregate lead, Bruno Fernandes striking the crossbar on 24 minutes when he spotted Sociedad goalkeeper Alex Remiro off his line.
James then drew a good save from Remiro with a headed effort following a lofted ball from Fernandes on 36 minutes.
Both sides made three substitutions at half-time, perhaps mindful of United’s four-goal advantage from the first leg and domestic interests.
With the game heading to its conclusion Shola Shoretire was introduced on 76 minutes, becoming United’s youngest-ever player in a European game at 17 years old and 23 days.