Australia captain Aaron Finch has backed Mitchell Marsh to shine if injured all-rounder Marcus Stoinis is forced to pull out of the World Cup.

Stoinis suffered a side strain in Australia's defeat against India last weekend and was sidelined when the holders beat Pakistan on Wednesday.

He has already been ruled out of Saturday's match against Sri Lanka at The Oval and his involvement for the rest of the tournament will be decided in the next few days.

"Stoinis won't be available again. He hasn't bowled yet in his recovery over the last couple of days," Finch told reporters on Friday.

"So having four days between this game and the next game, I think that will be the ideal time to really test him out and assess him.

"But I think over the next five, six days there will be a call made on that, just based on what he can and can't do.

"We've seen him batting and running, no problem. Just he hasn't tried to bowl yet. Just waiting for the injury to settle down a bit more."

Marsh is already on standby after flying to England to link up with Finch's squad and the skipper has full confidence in the understudy.

He was due to travel with Australia's A side to England this week but flew over a couple of days earlier as cover for Stoinis.

Marsh has played no professional cricket since March, while his last 50-over appearance was 18 months ago.

But if Cricket Australia does decide to trigger a formal request to remove Stoinis then Finch backs Marsh to be ready to face Bangladesh next Thursday.

"The short answer is yes. There is confidence that Mitch will come in and do well if selected, if Stoinis doesn't recover properly," he said.

Despite three wins from their opening four games, Finch admits Australia have yet to hit top form.

"I don't think we've gone anywhere near our best, which is still a good thing, that we've got six points on the board while not playing anywhere near our best cricket," he added.

"But as long as we're improving, and continue to go up and not go backwards, I think that's the key."

Feature image courtesy: AFP / Mal FairClough