The Black Cats currently sit second from bottom in the Premier League.
"I'd be kidding if I said the players we're going to bring will massively make a big difference," said Moyes.
"We probably couldn't get that level of player and probably wouldn't have the finances."
He added: "To suggest the players we bring in would be making a big difference, I think, wouldn't be correct."
Moyes refused to confirm any truth in a reported £10m move for Championship Norwich midfielder Robbie Brady.
"I think we have been linked with about four of five players this week and I can tell you that of the players we're in for, it's none of them," added the former Manchester United boss.