Arsenal v Borussia Dortmund: key clashes in the Champions League Olivier Giroud v Mats Hummels France forward Giroud has been central to Arsenal's resurgence this season, scoring six goals including one in the last European victory over Napoli. However, German defender Hummels, who came through Bayern Munich's academy, will be out to shackle his impact and limit Giroud's ability to bring Arsenal's formidable midfield into play. Mesut Özil v Jakub Blaszczykowski German playmaker Özil will be no stranger to the Dortmund side, but just how to stop him is another matter - as Napoli found out to their cost in the Gunners' last European outing. Özil was again instrumental in Saturday's 4-1 Barclays Premeir League win over Norwich with a rare headed goal. Poland midfield enforcer Blaszczykowski will be the man charged with keeping close tabs on Arsenal's £43 million man and nullify the supply chain. Per Mertesacker v Robert Lewandowski Giant German centre-back Mertesacker has grown in stature since signing for Arsenal from Werder Bremen in August 2011 and his partnership with Laurent Koscielny is keeping captain Thomas Vermaelen out of the side. However, that understanding will be tested by roving Pole Lewandowski, who will be out to prove his indifferent display against England at Wembley was a one-off.