Duncan has signed with Victorian NPL club Melbourne Knights after a trial on the weekend.

The towering centre-back or right-back previously played with Northcote City and Bentleigh Greens in the competition.

The former Jets and Mariners Youth player returned to Olympic from Lambton Jaffas last season and featured 19 times for the club, which changed its name from Hamilton to Newcastle over the summer.

Coach Peter McGuinness was disappointed to lose Duncan so close to the season, which starts for Olympic against Charlestown Azzurri away on March 21.

"It came out of the blue," McGuinness said.

"He rang last week to say an opportunity had come up to trial down there, which he did, and now they've offered him a spot.

"If you think you are settled with what you've got, and you don't go looking for anything else, then someone goes ... it's no different to someone getting injured for the season."

Despite the setback, McGuinness was confident in his defensive stocks.

"Tom Stewart, Kyle Hodges, Reece Papas, Reece Pettit, Jakob Cresnar and Cody Lucas can play central defence and Cody can play right-back, and he did on the weekend, so we've got it covered, but still I would have had one more," he said.

Olympic finished seventh last season and have added Scotsman Anthony Marenghi, Blake Green, Tom Curran, Dino Fajkovic and goalkeeper Kane Runge, while already losing Scott Pettit (Jaffas), Jarryd Sutherland (Weston) and keeper Tyler Warren.

Marenghi, though, suffered a groin strain in a trial and is in doubt for the start of the season.

Olympic have lost 8-0 to Wollongong and 7-2 to Sydney Olympic in trials so far and face the Jets Youth side on Sunday at 4pm at Darling Street Oval.