Cox will break an AFL record against St Kilda on Saturday by going past Stephen Milne for the most games played by a rookie-listed player, with 276.
The six-time All Australian will also equal the Eagles' games record currently held by dual premiership player and four-time club champion Glen Jakovich.
Emerging ruckman Sinclair is in awe of Cox's durability and brilliance.
"I'm in shock sometimes," Sinclair said.
"He's one of those guys who's good at everything. He's very professional around the club and takes his footy very seriously. He takes pride in his body.
"He's a freak.
"I think we're going to have to cart him out of the joint. I don't think he'll be going any time soon. He's playing terrific footy."
Cox, 32, has been a source of inspiration for Sinclair, who himself has been promoted off the rookie list.
"I looked at Coxy's career - he probably struggled a little bit when he first came to the club, as he told me. He did have to wait and be patient," Sinclair said.
"It's actually a really good story, for someone like him to come off the rookie list and he could arguably be the best ruckman of all time."
Sinclair, 24, was rookie drafted last season after moving from VFL side Port Melbourne to play for Subiaco in the WAFL in 2012.
He played five matches after being promoted to the senior list in 2013 and was retained as a rookie for this season. He was then promoted to the senior list ahead of round one due to the long-term injury suffered by Tom Barrass.
Despite the Eagles boasting the game's best ruck double act in Cox and Nic Naitanui, Sinclair has been given his opportunity by new West Coast coach Adam Simpson, who has elected to play a trio of ruckmen.
"I remember having a talk to 'Simmo' throughout the pre-season," Sinclair said.
"We had a bit of discussion about the potential of playing three talls in the side.
"One thing Simmo wants is flexibility from our rucks, to be able to go forward and contribute.
"Some of it is quite structured. But as the game goes on, if you're tired, you'll go down and have a rest down forward. If you're feeling pretty good, you'll go in the ruck."
Sinclair believes he needs to impact the scoreboard more if he is going to keep his spot in the side. He has kicked 1.3 in the first two games after kicking 3.5 in his five matches last year.
"I think I need to capitalise on my opportunities," Sinclair said.
"I'm getting shots on goal, I probably just need to start putting them through the sticks. That's probably the main thing, and that flexibility that I spoke about before."
