It's been successful, which is fine, but we should be able to adapt during the course of the game.
It's only a matter of changing people 10 yards in, 10 yards out. There's not much more to it than that."
Celtic is also in talks with Paderborn for midfielder Mika Baur and Frosinone winger Fares Ghedjemis, who scored 15 goals last season.
O'Neill explained the financial gap between Scottish football and top European leagues when pursuing transfer targets.
"I've got plenty of confidence that we can get the players in," O'Neill said. "Obviously, we're hoping that.
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It's not easy. If some good players go, you're trying to replace them.
We're a selling club because of the finance. The finance here in Scotland is just not there.
And I think, way back 25 years ago, when I was able to sign the likes of Sutton and Hartson, I think the gap between that and an English Premier League wasn't as great.
I could be able to get those players up. Actually, they wanted to come.
But it is difficult.
And the difficulty is letting some of your best players go and trying to replace them with, obviously, less revenue and trying to make them as good or, eventually, better than what you let go.
That, eventually, has its problems."
The manager confirmed he has held direct talks with prospective signings during a recent trip to London.
"Yes, we actually have spoken to other targets," O'Neill said.
