Some Arsenal fans on twitter are calling this a sign of the end of Wenger, as Mavropanos was identified and pushed by Sven Mislintat (formerly of Borussia Dortmund, who also is at least partly responsible for Sokratis joining that club). It's implied that Wenger didn't even know who he was, which is why he was so surprised by him during training.
It's a great sign that he's being kept on until the end of the season, but it remains to be seen if he'll see any time. Arsenal has some problems defensively, including the pending retirement of Mertesacker, so expect a big signing next summer. Mavropanos will have to beat out several young prospects that are currently at the club (but who have failed to make a case for themselves).