"I think in his first spell at Motherwell he was probably the best box-to-box midfield player - maybe not just in Scotland but in the UK. He was a Steven Gerrard-type player. It was no surprise he joined Celtic."
"But for injury, I'm sure he would have had many, many more caps," said Brown, who gave O'Donnell his only international appearance.
Scottish FA Chief Executive Gordon Smith said the death was 'absolutely devastating news'.
"Phil was not just a wonderful footballer, he was a great human being," added Smith.
Meanwhile, O'Donnell's manager at Motherwell, Mark McGhee, said that his whole team are in shock.
"We are all totally devestated and our thoughts are with Phil's wife and family. Phil was a great professional and gave everything when he played the game - this is a shocking tragedy."
Team mate Chris Porter also offered an emotional tribute to the 35 year-old.
"He was an amazing guy. When I first came he was the first person I met. He came out especially to meet me.
"It's a massive loss to Motherwell. He was just a great player and a great person. Everyone will miss him."