The Black Cats are sliding into the relegation battle and lost defender Carlos Cuellar in Sunday's 1-1 draw with Norwich, and O'Neill must now see his big stars travel to play for their countries.
The Ulsterman said: "It will be a worry for us because we don't have that many to choose from. That's been a situation that has appeared all season, so I would obviously be concerned about the players going away.
"Some of them have two matches to play as well - a lot of them have - so I will just wait to see when they come back again.
"With Carlos Cuellar getting an injury, our concern is the players. We just need every able-bodied person around for the last eight games."
On being held by the visiting Canaries on Sunday, despite playing much of the game with a man advantage, O'Neill added: "The fact is that we didn't create clear-cut chances in the game.
"Yes, I know we had a number of balls go into the box that were cleared first time by their defenders when we should have been in there competing for that, but overall, that would be just the point, that we didn't create enough in the second half."