Yes, it is still a regurgitation- I went back & amended part of my earlier answer, having reconsidered this. He obviously digested most of his prey- all that was left was "the hairball", but with an abundance of caution (since you did experience the sourness smell of a regurge)- I'd at least wait 2 weeks & feed on the smaller side at next meal, just in case his digestive enzymes were at all to blame, or are now a bit lacking after getting rid of "the hairball". Adding digestive enzymes might help some, or might be unnecessary. no way to know- it's your decision. If you already have Nutribac on hand, I would add that, as it won't hurt & may help.