They've already said that they cannot get FORSTER back because it isn't in the loan agreement with Celtic, and they've ruled out bringing Given back to St.James' Park on an emergency deal, so unless they have other targets in mind, I think they're sticking with Krul.
No need for an emergency or to cancel Forster's uninterruptable loan. This will be good for Krul's development while Forster gets even more first team action at a decent level.