SYDNEY, AUSTRALIA - APRIL 07: Sam Reid of the Swans gathers the ball during the round three AFL match between the Sydney Swans and the Greater Western Sydney Giants at Sydney Cricket Ground on April 7, 2018 in Sydney, Australia. (Photo by Brendon Thorne/Getty Images)

Sydney were forced to make their medical substitution in the second half of Saturday's preliminary final against Collingwood, with veteran Sam Reid suffering an adductor injury.

The incident leading to the injury is not yet known, however Reid's absence opened up a vacancy in Sydney's attack as they look to defend their five-goal half-time lead.

The change was confirmed within the opening minutes of the third quarter, with young flanker Braeden Campbell brought into the game.

Sydney coach John Longmire revealed he wasn't certain on the severity of Reid's injury, while 7 News' Tom Browne stated the club were hopeful their veteran tall might be fit for next weekend's grand final clash.

Meanwhile, Collingwood ruckman Mason Cox was forced down to the race midway through the third term with a groin injury. The setback could spell season over for Cox even if the Pies book themselves a spot in the grand final.

Key-position tall Nathan Kreuger has replaced Cox.