That's why we have Balotelli and probably getting Arnautovic as well. Cassano will want to start gaames all season long(not only bcz of World Cup season) not just in January! Balotelli also wants his chance to get into Italy's WC squad. If Cassano arrives, he becomes 4th choice and probably will ask to leave! We can't get Cassano now for many reasons, unless we don't use him as a forward except for times where either Milito or Eto'o or Balotelli is unavailable. Cassano is great and all, but he's no Maradona, Romario, Ronaldo, Zlatan or let me say it otherwise, a man one team.
If Mourinho can keep Mario happy, and utilise Eto'o, Milito and Cassano without anyone getting unhappy or whatever, then i'd be behind this transfer 100%. But I can't see how we can manage a stability within the squad if we do get him, no matter if we offload the dead wood as well.
Anyway, I doubt we'd spend €10m on Arnautovic to be our 5th striker! Cassano's transfer would seem illogical and unnecessary if that goes through.