Rooney also believes Amorim's youth could be an issue as the Portuguese battles to get his team to start winning.
"His words were damning on the players," said Rooney. "If your manager comes out with those words, there's something broken.
"What we forget is he's only 40. He's a young manager.
"Manchester United is so different to where he's been before. You come in and there's the media team, the commercial team, the fans and the media.
"There's a pressure with United, which he will never have felt before. When you're not winning games, that can start to get (to you) and it looked like he was at a breaking point."
On the prospect of Amorim being sacked, Rooney believes United may struggle to find a replacement.
He also told BBC Sport: "The problem we've got now is who would want to take it.
"It is crazy to say that, you've got one of the biggest clubs in the world and you've probably been just sitting there thinking… not now. How quick it's changed."