Hawthorn defender Brent Guerra has ruled himself out of Saturday's AFL grand final.

Guerra injured a hamstring in the last home and away round and had been battling to regain fitness in time for Saturday's decider against Sydney at the MCG.

But he told coach Alastair Clarkson and then teammates on Monday that he knew he had run out of time.

He said while desperate to play, he couldn't guarantee his body would hold up, so wanted to remove any risk or distraction for the club ahead of their most important game of the year.

"As desperate as I am to play on Saturday, I realised that I couldn't guarantee to Clarko or the team that I could do everything that would be asked of me in a grand final," Guerra said.

"It was a gut-wrenching decision to make and it was difficult to address my teammates and coaches today.

"But they know they have a job to do on Saturday and that needs to be their only focus right now."