We also need to keep one of our full backs back all the time, while the other needs to attack (to keep their winger occupied). As you say, I'm worried by Lukaku, he has the pace, power and skill to hurt us if they boot a long ball to him and we're not concentrating. Not over yet.
I'd like to see us bring in a new DM, have coquelin as backup and Arteta as a squad player, with his age and injuries it's inevitable he will play a smaller role but I think he's worth keeping around.
Koscielny has developed this habit of putting his own defence under pressure with poor passes. He needs to just play things simple.
Good play, patient build up, clever lay off by giroud, didn't get the finish but Cazorla did well during the build up