O'Shea will today name his squad for the friendly double header against Hungary and Portugal, a squad without senior players such as Gavin Bazunu, Nathan Collins, Choedozie Ogbene and Evan Ferguson.
Waterford-born O'Shea has received no assurances from the FAI regarding their long-term plans to replace Stephen Kenny but Duff, speaking to former team-mate Richie Sadlier on the Second Captains podcast, was scathing of the process.
'If I were him, I wouldn't do it a second time. They left it outside to dry. He's just hanging around. 'I understand, don't I?'” Duff told Second Captains.
John O'Shea
'He's too good a person and too good a coach to do that. I hope he doesn't take it the wrong way, but I would encourage John to go on his own journey and coach every day because that's where the learning lies.
“I was so happy for John that he could govern his country. It's something I may never do, and it's the pinnacle of any proud Irishman's football career.
“But the whole process was absolutely embarrassing. We used to be a football power; we are no longer.
“How the FAI is run, the whole organization from top to bottom, I think they should suffer from that. John, if he's going to do it again, great, maybe he'll get it full-time, (which) is absolutely great.
“I'd say he was so proud standing on the sidelines of the Aviva, but if I were him I wouldn't put up with it a second time.”
Duff, who guided Shels to Premier Division leadership, encouraged O'Shea – who has coached at club level as well as with the Irish U21 and senior teams, but has never directly managed a team apart from his spell as caretaker with Ireland – to start at club level.
“We are young coaches. He will be a brilliant coach, a brilliant manager. I'd like to see John get a club team now. And don't wait to see what the FAI does,” Duff added.