Arsenal's Flamini out of Man Utd clash
Flamini sustained the injury during Tuesday's heartbreaking Champions League quarter-final defeat by Liverpool and is expected to be sidelined for three weeks.
Brazil international Gilberto, who replaced Flamini in the 43rd minute at Anfield, will also deputise for the trip to Old Trafford.
"It's a big blow but Gilberto has a lot of quality and experience and he will come in. Gilberto can help the team recover and focus."