The Mirror reports that the Spanish midfielder tore a muscle in training and could miss the start of Arsenal's Premier League campaign as well as their Champions League play-off against Fenerbahce.
Bacary Sagna, Aaron Ramsey and Theo Walcott are also all currently carrying knocks but still Arsenal have signed just one player this summer despite a number of departures.
On Thursday, manager Arsene Wenger maintained the club will continue to be active in the transfer window right up to the deadline - but stressed he could give no guarantees of how many new players the club would be able to sign before September 2.
Chief executive Ivan Gazidis may have talked up the Emirates Stadium club's new "financial firepower" at the end of last season, but so far the only confirmed signing has been France Under-21 forward Yaya Sanogo, and that on a free transfer from Auxerre.
Arsenal, who failed to land Real Madrid's Gonzalo Higuain earlier in the summer, continue to pursue wantaway Liverpool striker Luis Suarez, but will have to up their bid to closer to £50 million with the Reds unwilling to sell.
Bayern Munich's Brazil midfielder Luiz Gustavo is a target but has been tipped for a move to Wolfsburg, while 20-year-old Lille winger Florian Thauvin is another said to be on Wenger's radar, as is Romania forward Ciprian Marica following his release by Schalke.
While the Gunners boss clearly is determined to bolster his squad, albeit not in time for next week's crucial Champions League play-off first leg against Fenerbache in Turkey, Wenger accepts patience will be key as the clock ticks down towards the September 2 deadline.
"We are active," Wenger told Arsenal Player. "We are in a situation like Manchester United, like Chelsea, where everybody expects signings and at the moment not a lot has happened.
"There are 18 days to go, so that is a long time in the transfer market and we will be active. I cannot certify what the number [of players coming in will be], but we will be active until the end, that is for sure."
Although Wenger may have so far been frustrated in his plans to strengthen the team, at least several fringe players have been taken off the wage bill, with the exits of Gervinho and Marouane Chamakh both now confirmed.
Wenger feels Ivory Coast forward Gervinho, who signed for Roma in an £8 million deal, had been unable to overcome the negativity from some Arsenal supporters.
"I made that decision because he looked to play with a lack of confidence, especially at the Emirates," Wenger said.
"He is a very creative player, an instinctive dribbler and for that you need a lot of confidence to be completely efficient.
"I felt that in the last six months it was very difficult for him to express his talent in a confident way.
"Then you sit there and think, 'do I bring him back or does he need a new challenge to get that confidence back?' and I did not want to stop him from getting a new chance."