"You can't rely on one particular player over a long period," Cox said. "But he's really important."

It would be unfair on Sinclair to say his value is tied solely to the few options around him as that would be ignoring the considerable contribution he has made this year - his first as a No.1 ruckman.

What had appeared a potential weakness for the Swans has become a positive. He has played a big role in the key wins against Greater Western Sydney and Geelong.

Last week he polled the maximum 10 votes from the coaches after eclipsing Carlton's Matthew Kreuzer. Leading All Australian contenders Max Gawn and Brodie Grundy are the only rucks above him in the AFL coaches award, placing him ahead of Stefan Martin and Nic Naitanui. He would most likely be in the top five in the Swans' best and fairest voting right now.

Sinclair, at 200cm, does not have the height to be a dominant rap ruckman but he covers the ground well for a man his size.

His average of 16 disposals a game places him in the top five for rucks, and no ruckman has kicked more than his seven goals this year.

"If you're talking from a rucking point of view he won't win the percentage of hit-outs like a Max Gawn, Aaron Sandilands, Brodie Grundy or Nic Naitanui, for him it's about being predictable to our midfielders to make sure he's giving a contest that allows us to get an advantage at ground level," Cox, a six-time All Australian ruckman, said.

"He has to make up in other areas - his follow up work, he's had games where he's had a lot of clearances."

Sinclair was dominant in two games last year, including a five-goal haul against this week's opponent St Kilda, but there was a big gap between his best and worst.

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The same could be said of his marking. He could take a towering pack mark but also put down straightforward chances on the lead. Through hard work - a trait which earned him the best clubman award last year - those mistakes have become rarer.

"His diligence and attention to detail to be the best player he can is as good as I've seen in football," Cox said.

"He leaves no stone unturned. He's done the work through the week and can sleep well knowing when it gets to games he can have an impact. He realised he could be inconsistent so he put the work in.

"Marking is a repetitive skill you have to continually practice. I see it as a strength in his game. When he's having three or four contested marks in games, it's telling.

"For him to stand up and do what he's done is a credit to him. It's exciting what the second half of the year can hold."