Arnold has been one of Newcastle’s stand-out performers this season since the Irishman arrived from French second-tier side Brive in the summer.
While understanding that injuries are part of the game, Diamond is unhappy at the manner in which Arnold has been ruled out of his plans.
“The game is hard enough as it is without doing things like that,” he said.
“The young lad was frustrated, they’re losing a game they’re not expecting to lose and in the 80th minute, he’s seen an easy target and effectively smashed his (Arnold’s) knee.
“It looks dreadful on the video. There was everything wrong about it, coming in from the side, targeting the knee. It’s frustrating.”
