Arsenal manager Arsene Wenger ruled the Frenchman out earlier in the week, saying: "Lacazette has a groin problem and he will be out certainly for Saturday's game."
Despite Wenger's certainty, Lacazette was a shock inclusion in the Gunners lineup for the clash at the Emirates Stadium.
Lacazette's inclusion means Arsenal are unchanged from their 5-0 win over Huddersfield on Wednesday.
For the Red Devils, Anthony Martial is preferred to Marcus Rashford as United are also unchanged. After suffering a minor injury in United's 4-2 win over Watford on Tuesday, Nemanja Matic is back in the line-up for Jose Mourinho's men.
Marouane Fellaini, Phil Jones and Eric Bailly all were unable to travel to London due to injuries.
Arsenal XI: Cech; Bellerin, Koscielny, Mustafi, Monreal, Kolasinac; Ramsey, Xhaka; Ozil, Alexis; Lacazette. Subs: Ospina, Mertesacker, Coquelin, Wilshere, Iwobi, Welbeck, Giroud
Man Utd XI: De Gea, Valencia, Lindelof, Smalling, Rojo, Young, Matic, Pogba, Lingard, Lukaku, Martial. Subs: Romero, Darmian, Blind, McTominay, Herrera, Mata, Rashford