Seabold pitched 6 innings yesterday, so he's out.
My guess would be Richards will get the start. He's the best bet to go multiple innings.
Edit - Looks like the Sox called up somebody named Kutter Crawford; he's a starter with Worcester, so I'll assume he's getting the nod. Okay numbers at Portland in 10 games, not so great at Worcester in 6. He's only thrown 75 innings this year so far, but all but one appearance have been starts.