Diaby will most likely be replacing Cesc if Cesc is out. With the way Arsene was praising Diaby about how similar he is to Vieira and how this could be his year post match Pompey I'm guessing he is almost a dead cert to start the Man Utd game. In fact I think Diaby will start even if Cesc is fit. Wenger has always been known to field lineups with the intention of controlling the midfield in difficult and probably physical away games such as Man Utd at Old Trafford.

Probable lineups IMO.

With Cesc:

-----------------Van Persie-------------------

Diaby---------------------------------Arshavin

-----------Denilson-------Fabregas-----------

--------------------Song------------------------

Clichy----Vermaelen----Gallas--------Sagna

-------------------Almunia----------------------


Without Cesc:

-----------------Van Persie-------------------

Arshavin------------------------------Bendtner

------------Diaby-------Denilson-------------

--------------------Song------------------------

Clichy----Vermaelen----Gallas--------Sagna

-------------------Almunia----------------------