The Eagles slipped to their fifth successive league defeat in Monday's London derby, despite taking the lead early on through Adrian Mariappa.
Holloway recruited heavily in the summer, but has now conceded that signing 15 players so quickly has left Palace struggling to find the morale that formed the basis of their promotion success last year.
He told the South London Press: "We've lost five games since we've announced the 25 and I feel there is something wrong there. I don't feel we've got the same spirit.
"Maybe we changed too many. But that squad is who we've got until January. So they better buck up and stick together, and learn how to stick together. Because it's tough. And we've got a nice easy one coming up next, which is Arsenal.
"You're trying to gel a group of people together. We've made 12, 13 new signings and that's out of 25 people. It's not excuses. It's just the way it is. And at the minute every single thing that could go wrong has and is."
Palace sit second from bottom with one victory from eight league games ahead of this weekend's clash with leaders Arsenal.