Wenger has insisted that he is hopeful that his two defenders will be able to return in the near future, but he wasn't so optimistic regarding Ramsey, who was introduced as a second-half substitute before going off injured during the closing stages.
The Frenchman said: "I am a bit in shock because we have lost our two centre-backs and Aaron Ramsey.
"Mertesacker is not serious - it's a concussion. Gabriel looks alright but I am more concerned about Ramsey - it looks like a muscular thigh injury."
Ramsey would have been hoping to continue his return to form after scoring against Tottenham Hotspur at the weekend, but he looks set to miss crucial fixtures in the Premier League, FA Cup and Champions League.
It remains to be seen what the prognosis is regarding Mertesacker and Gabriel, but if Mertesacker is diagnosed with a concussion, he is unlikely to be available for the tie with the Hornets at the Emirates Stadium.