The Magpies have lost their first two games of the new Premier League season, failing to score against both Tottenham Hotspur and fellow Championship promotees Huddersfield Town.
Ex-Newcastle midfielder Barton told talkSPORT: "They do need some investment. When you come up, you do need investment, but there is also a case that Benitez needs to get on with it.
"You've got what you've got. Okay, you maybe need to keep it private in terms of [asking] can we get bodies in, but he's getting paid an awful lot of money, so coach what you've got in front of you.
"It doesn't help when the manager is out there moaning, saying he's not happy with the situation. That definitely festers out on the pitch.
"If you're a player and you hear the manager moaning about investment it doesn't inspire confidence because he's saying they're not good enough."