Duff is preparing his Cheltenham Town side for a two-legged League 2 play-off semi-final against Northampton Town, with the first leg at the Sixfields Stadium on Thursday, June 18th, before the return at the Jonny-Rocks Stadium on Monday, June 22nd.
Duff helped the Robins to fourth place in his first full season in charge, after the campaign was brought to a conclusion on points per game.
Sinclair and Duff both signed for the Clarets in the summer of 2004, Sinclair arriving as a veteran campaigner after long spells with Chelsea and Leicester, and Duff as, what Steve Cotterill famously called "a calculated risk" from Cheltenham.
The pair alternated between right back and centre back, before the former left for Huddersfield Town in 2007.
Duff would remain on the playing staff until 2016, winning three promotions to the Premier League, before becoming Under 18s and then Under 23s boss at the club.
And Sinclair, who was helping out at fellow League 2 side Port Vale, doing some work with the defence, before lockdown, said: "I think, if anybody had looked at the level he'd come from with Cheltenham, to come and play in the Championship, it's a big step up.
"But he was athletic, a hard worker, and he took on board the experience around him in the team.
"For him to go on and play 12 years at the club is incredible - he would probably be the first to admit he over-achieved, with the level he'd come from.
"He was terrific for the club.
"I still speak to Duffo every now and again, we had a conversation before Cheltenham played at Port Vale on the phone, and he's done terrific, working in the academy at Burnley and getting the opportunity to manage himself.
"i didn't really see him being a manager when he was younger, but I'm not surprised because he's been heavily influenced by Steve Cotterill throughout his career, so he probably took a lot of his habits on, so it did make sense.
"I did one of my youth modules at Burnley with Duffo, my two and three modules, so I knew he was on that ladder, and wasn't surprised he got the opportunity at Cheltenham with his history with the club.
"It made sense, and he's done terrific."