Diame played with Serb at Newcastle.
"When you see the love Mitro had from the fans, I think he badly wanted to stay at Newcastle and enjoy every weekend with the fans because he had a very strong connection with them," Diame told ChronicleLive.
"He was sad to leave, of course, but, as a football player, you always want to play so he had to go. You can see he's still scoring goals everywhere so I'm happy for him.
"Maybe some people have got doubts, but people who have been with him in the dressing room don't have doubts when training with him every day about what he can do. If there's one striker you don't want to play against, it's Mitro.
"I remember the song Newcastle fans had for him - 'Mitro's on fire, your defence is terrified' - and it's real. He scored plenty of goals in the Championship and him scoring in the Premier League is not a surprise. It's all about confidence and the manager [Marco Silva] put a lot of faith in him. He's in a nice place."