Iwobi gave Unai Emery's side an early lead at the Roazhon Park in the third minute, however, Sokratis Papastathopoulos' expulsion in the 41st minute after receiving two yellow cards reduced the Gunners to 10 men.
The Greek centre-back received a second caution for a dangerous play against Sarr and Benjamin Bourigeaud took advantage of the resulting free-kick to put the Ligue 1 outfit levelled moments before the end of the first 45 minutes.
Nacho Monreal's own goal in the 65th minute turned the game around to favour the hosts before Sarr compounded Arsenal's woes with a diving header past Petr Cech in the 88th minute.
The 21-year-old has now scored four goals in eight Europa League matches this term.
Arsenal now face an uphill task to overturn the defeat next Thursday at the Emirates Stadium to revive their chances of winning a silverware this season.