GIANTS defender Zac Williams has been ruled out of Saturday's crucial clash with Geelong with a hamstring injury, opening up an opportunity for debutant Jeremy Finlayson.

Williams trained with the rest of his teammates at their main session on Wednesday at Spotless Stadium, but pulled up sore and was ruled out of the game on Friday morning.

The loss of Williams opens the door for GIANTS Academy product Jeremy Finlayson, who will make his debut.

Taken with selection 85 at the 2014 NAB AFL draft, the athletic backman has been in brilliant form in the NEAFL, averaging 27.8 possessions in his six games, and could have been picked sooner had it not been for a mid-season groin injury.

The 21-year-old won the GIANTS' NEAFL development award and starred in the club's reserves premiership team last year.

"One goes down and another one comes up, it's a fantastic story with Jeremy playing his first game," coach Leon Cameron said on Friday.

"It's a huge challenge for him because he steps into an A-grade game first up against the Cats, but he's been in some good form.

"He's been knocking on the door so he deserves his spot. It's one of those things where you just want to see how he fares, because the confidence he shows at the lower level is fantastic, and we feel like he can transfer that to AFL level.

"We took him as a raw young kid at the back end of the draft three years ago and he's taken a few steps forward and a couple back, so we're really rapt that he's done a long apprenticeship and finally gets his opportunity."

Star midfielder Josh Kelly (hip), forward Devon Smith (knee) and big man Rory Lobb (groin) all trained on Friday and will add enormous strength to the home side, who face a fellow top-four team for the first time since they defeated Port Adelaide by 31 points in Canberra in round four.

Kelly adds even more star power to the GIANTS midfield, with Dylan Shiel, Callan Ward, Tom Scully and Lachie Whitfield set to take on Geelong's gun pairing of Joel Selwood – if he overcomes a bout of concussion – and Patrick Dangerfield.

Cameron used Sam Reid in a shutdown role on Brisbane Lion Dayne Zorko with great effect last week, but he remained tight-lipped when asked about throwing a tag at the Cats on Saturday night.

"There's that many of them it's very hard to pick just one," Cameron said.

"You've got (Joel) Selwood, (Patrick) Dangerfield, (Cam) Guthrie, (Mitch) Duncan, you can look across all of them.

"There's no doubt 'Reidy' (Sam Reid) will roll through the midfield, but whether he has a hard tag or not, we're a bit unsure at this stage."

The GIANTS take on Geelong in a blockbuster Saturday night clash at Spotless Stadium, click here to get your tickets now.