Arsenal manager Mikel Arteta says the club did a "phenomenal job" trying to make another signing before the transfer deadline.
But Arsenal were unable to secure a new midfielder, with Aston Villa turning down two bids for Douglas Luiz.
"We tried to act but we didn't find the right player," said Arteta.
"You see the amount of departures compared to the recruitment and again the club has done a phenomenal job and put everything on the table to try to bring in the players and profile that we wanted," added the Gunners boss.
"We did the business pretty early, but unfortunately in the last 72 hours Thomas got injured in the same place as last time and then Mo has a long-term injury and we had to react in the market.