Laurent Koscielny was Arsenal's best centre back last season; a constant starter who sometimes single-handedly kept Arsenal from being a complete and utter disaster defensively last season. He's probably Arsenal's best centre back, but has had his playing time limited this season because Thomas Vermaelen has been fit all this season, and is also Arsenal's captain. It's unfortunate, because Koscielny has all the attributes of Vermaelen, while being better, and less culpable to the constant mistakes that Vermaelen makes.
He showed some of that against Bayern Munich in the first leg, where he kept it from being a rout, and he definitely showed that yesterday. Not only did he score the goal that made Arsenal fans have high levels of hope, but he, along with Per Mertesacker, kept Bayern out at the back. There was the classic Koscielny moment, where he chased down Thomas Mueller and made an excellent tackle. He made 4 interceptions and 1 tackle, and made 14 clearances, showing how he was in the right place almost every single time. Mertesacker was too, and such a performance is encouraging, as long as Arsene Wenger plays his two best centre backs and consigns Thomas Vermaelen to the bench.