Ryan Harris believes a fit and firing Shane Watson could be just the tonic for Australia's overworked bowling attack.

Watson was set to bowl five overs at training in Cape Town on Thursday, his final fitness test before selectors mull the best XI for the series decider that starts tomorrow.

The 32-year-old, told he needs to bowl to warrant national selection during a layoff with a calf injury, would bring much-needed respite to Harris, Mitchell Johnson and Peter Siddle.

The trio bowled a combined 133 overs in the Second Test - the highest mark in a Test for three Australian pacemen in three years.

"Having Watto back would be huge, having that extra bowler," Harris said.

"We've seen when Davey [Warner] has to bowl a few here and there and Michael [Clarke] has to fill in - they're bowling Watto's overs."

Watson is tipped to be drafted into the side at the expense of either Shaun Marsh or Alex Doolan.

"Who he comes in for I don't know. The guys who are already in there, Doolan and Marsh, didn't go well last game but have done well," Harris said.

"But I think we need Watto playing. It's just a matter of who he comes in for."

Marsh and Doolan both made runs in Australia's First Test win before floundering in the Second Test loss.

Coach Darren Lehmann has said it would be "really unlucky" for a batsman in the side to be dropped, but conceded Watson's overs add another dimension to his team.

"As long as he's bowling overs and he's fit, we would love to have that extra bowler," Lehmann said after his side slumped to a 231-run loss in Port Elizabeth.

AAP