I'm not sure Lansbury is going anywhere, strange that he hasn't actually had that much game time but Wenger was pretty unequivocal about his involvement with the first team, that was back when he was hopeful of keeping one of Cesc or Nasri though so any experienced replacement might push him out, then again it might push Rosicky out instead, that would be my preference.
Anyway, we need a midfielder. Possibly two if Diaby isn't fixed after his last op.