LONDON - Arsenal defender Johan Djourou is doubtful for Wednesday's Champions League quarter-final second leg against Villarreal after damaging his knee playing in Saturday's 4-1 win over Wigan Athletic. The loss of the 22-year-old central defender is a blow Arsenal could have done without as they will also be missing defender William Gallas, injured in last week's 1-1 first leg draw in Spain. "Johan Djourou has a knee problem and that's the bad news of the day. It looks as though he'll be out for Wednesday," Arsenal manager Arsene Wenger was quoted on the club's website. "We lost two central defenders in two games, we have a few more games to go and we haven't got enough of them to lose one every game." Mikael Silvestre, back to full fitness after not starting a League game for Arsenal since December, replaced Djourou after 35 minutes and is likely to play on Wednesday. Villarreal also have also been hit with injury woes after midfielder Marcos Senna was ruled out of the match with a strained a muscle in his left leg. The Spain international, who scored Villarreal's goal in the first leg, had to leave the field with 10 minutes left of Saturday's Primera Liga match at home to Malaga, which they lost 2-0.