Moyes' side have won just two of their last six matches in all competitions and have made their worst start to a league season since 1989.
Despite their inconsistent form in the Premier League, United have performed well in the Champions League, having followed a 4-2 home win over Bayer Leverkusen with a 1-1 draw at Shakhtar Donetsk in October.
Wayne Rooney's strike against Leverkusen means he is now Britain's all-time leading scorer in the Champions League with 29 goals.
United have lost their last two home fixtures against Spanish sides – to Real Madrid in the round of 16 in last season's Champions League and to Athletic Bilbao at the same stage of the 2011-12 Europa League.
Tom Cleverley (calf) and Rio Ferdinand (groin) missed Saturday's draw against Southampton and are in doubt for Wednesday's match.
Real Sociedad registered a much-needed win on Saturday, a 2-1 victory at Valencia ended a winless run of eight matches in all competitions.
That result lifted them to 11th in La Liga and highly-rated French attacker Antoine Griezmann – who scored in the victory – hopes it inspires them to an Old Trafford upset.
"We're not going (to Manchester) merely as tourists. We are going as a quality side who will be trying to win three points that will help us to continue in the tournament," Griezmann said.
"We will analyse what we did well last Saturday and look to repeat that."
Real Sociedad have lost both of their Champions League matches, with a 2-1 defeat at Leverkusen earlier this month particularly heart-breaking, as Jens Hegeler scored a stunning free kick in stoppage time for the winners.
They need a victory and will be thankful midfielder Xabi Prieto returned from a thigh injury on Saturday.
Midfielder Esteban Granero – who is on loan from QPR – is a long-term absentee with a knee injury.
MasterCard is excited to present episode one of the Priceless Memories UEFA Champions League video series. See what Liverpool legend Didi Hamann has to say about the infamous 2005 final!