Ole Gunnar Solskjaer's United are all but into the last 16, with four 'away' goals to take into the return game in a week's time, after the first leg was played in Turin due to coronavirus travel restrictions.
United started slowly in the first 10 minutes but began to grow into the game, with Marcus Rashford wasting two excellent chances.
The Premier League outfit did grab the lead in the 27th minute when Fernandes pounced on a mix-up between Sociedad goalkeeper Alex Remiro and defender Robin Le Normand, as they attempted to clear Rashford's clipped pass, to slot in.
February 18, 2021
Solskjaer's men doubled their advantage 12 minutes into the second period as Fernandes stroked home his 21st goal of the season in all competitions after Daniel James had laid off Rashford's ball. The effort was initially ruled out for offside before the linesman's decision was overruled by VAR.
United continued to play at a pace Sociedad could not deal with and Rashford finally got his goal in the 65th minute following a blistering counter-attack started by goalkeeper Dean Henderson.
The tie was surely put to bed in the final minute of normal time as James raced clear and fired into the bottom corner.