I'm guessing Gazzanigga, since there's a rule saying that if our two registered goalies are injured or suspended we can play an unregistered one anyway. So leaving him out just means we have to put Brandon Austin on the bench instead of him if one of Vorm or Lloris is unavailable for a match.
The injury has to be more than a 30 day one I think. So if it's only a minor we'd have to start Vorm with unproven keeper on the bench. We're not gonna put ourselves in that position. Gazaa will surely be in the squad. Foyth is seen as part of the future by Poch so it would be a massive snub to him not to be put in the CL squad. So I think it's between Sissoko and Lamela. I have my own thoughts which it will be.