draw on Tuesday if Bayer Leverkusen beat Shakhtar Donetsk in the group's other game in Ukraine.
United are top on seven points, Bundesliga side Leverkusen have six in second and Shakhtar are third on four.
Sociedad showcased their attacking prowess with a thumping 5-0 success at home to 10-man Osasuna on Saturday that lifted them to seventh in La Liga.
Defenders Ion Ansotegi and Inigo Martinez struck with headers either side of halftime before Osasuna centre back Jordan Loties was shown a straight red card for tripping Carlos Vela when he was through on goal.
In-form forward Antoine Griezmann nodded a third in the 56th minute, Gonzalo Castro finished off a sweeping break in the 82nd and substitute Haris Seferovic helped himself to a goal two minutes from time.
"It was the kind of match that every coach wants," Sociedad's inexperienced manager Jagoba Arrasate told a news conference.
"Winning, not conceding a goal, it wasn't easy but everything came off perfectly," added the 35-year-old.
United's win in London did not come without a cost and Moyes has injury concerns over midfielder Tom Cleverley and defenders Jonny Evans and Rafael, who all had to be replaced at halftime.
"Tom Cleverley had double vision, Jonny Evans's back stiffened up and Rafael went over on his ankle," Moyes said.
"I'm hoping they will be fit. Rafa is the biggest doubt. Tom should be okay in a day or two's time and Jonny, I couldn't tell how his back is.
"To go and play against any side in Spain who have qualified will be a hard test, but we will go over there and try and get a good result that will continue our good form in the group."