Joshua Zirkzee drilled in from just outside the area to give United the lead on 57 minutes in San Sebastian, but Mikel Oyarzabal levelled from the spot after Bruno Fernandes was punished for a handball.
Zirkzee cut a distraught figure after missing the decisive penalty in last week’s FA Cup shootout loss to Fulham and has endured a testing first season at United, whose only remaining hope of silverware is in the Europa League.
He scored for the first time in Europe this term, unleashing a sharp low drive from 20 yards after being teed up by Alejandro Garnacho’s inviting pass.
But Ruben Amorim’s side could not hold on to their advantage as Fernandes handled at a corner, with Oyarzabal confidently converting his spot-kick.
Andre Onana made two excellent saves to keep United level as Real Sociedad pressed for a winner, leaving the tie evenly poised ahead of next week’s second leg.
“I felt until the penalty we had control of the game and then I think the penalty changed a little bit the momentum,” Amorim said. “I felt our team in the last 30 minutes were really, really tired.
“We take this stage to Old Trafford, and it’s going to be a different game,” he added. “The pressure is going to be on us in that stadium and we have to be ready.”
Lucas Bergvall’s first-half own goal condemned Spurs to defeat in the Netherlands, and it could have been worse for Ange Postecoglou’s side were it not for two fine saves from Guglielmo Vicario.
Meanwhile, Rangers recorded an impressive 3-1 win away to a Fenerbahce team coached by Jose Mourinho.
Cyriel Dessers put Rangers ahead early on in Istanbul before Alexander Djiku hauled Fenerbahce level.
Vaclav Cerny restored the lead for the Scots before striking again nine minutes from time to place Rangers in firm control under interim boss Barry Ferguson.
Lyon also took a big stride towards the quarter-finals with a 3-1 victory at Romanians FCSB.
Paulo Fonseca fought back tears as his players celebrated Nicolas Tagliafico’s opener with him, a day after the Portuguese coach was banned from the dugout until November by French football officials after an angry altercation with a referee.
Malick Fofana won the game for Lyon with two late goals after Alexandru Baluta had equalised for FCSB.
Roma host Athletic Bilbao later on Thursday, while Ajax are at home to Eintracht Frankfurt. Lazio travel to Viktoria Plzen for the first leg of their tie. (AFP)
