Mitch Marsh has not lived up to expectations with bat or ball in the Test series against India but the all-rounder’s issues are form not fitness related, Australian head coach Andrew McDonald has conceded.

McDonald refused to declare Marsh a certain starter for the fifth Test starting on Friday but said Mitchell Starc was a “good chance” to back up for the decider at the Sydney Cricket Ground.

Marsh has failed to make an impact with the bat since compiling 47 in the second innings of the first Test in Perth. The 33-year-old has not reached double figures in his six other knocks in the series, compiling only 73 runs at an average of 10.42.

The medium pacer took three wickets but was hampered by a sore back as Australia were soundly beaten by India in Perth. Marsh has since bowled a combined 16 overs in three Tests without claiming another wicket.

“I think people have probably been reading too much into that [sore back],” McDonald said of Marsh. “We haven’t required him with the ball as often as what we would have thought.

“He bowled again, his speeds were up in the high 120s, there are no injury concerns there. To sort of head down that angle is a little bit unfair.”

In-form all-rounder Beau Webster was added to the squad as insurance after the first Test loss but is yet to debut for Australia. It would be a big call to hand Webster a first baggy green in Sydney at the expense of the reigning Allan Border medallist, though McDonald admitted Marsh’s lack of runs were a concern.

“Would he like better performances? There’s no doubt about that,” McDonald said. “Over four Test matches he hasn’t been able to deliver at the level that he would like and we would like. But he’s up and about. We just won a Test match. He’s in a pretty good headspace.”

Australia will wait to see how Starc recovers from an epic Boxing Day Test but are confident Jhye Richardson or Sean Abbott could step up if the star quick is ruled out. After a stunning MCG Test that went into the last hour on day five, Australia and India have just three days off before the fifth and final Test begins.

Starc struggled with sore ribs on day three in Melbourne, but bravely bowled on and claimed the critical wicket of veteran Virat Kohli in the last over before lunch on Monday.

“Any time you get through the game, it’s always a good indicator that you’re a chance at the next game,” McDonald said of Starc. “It didn’t stop him. Clearly there was a little bit of discomfort early on in spells, but once he got warm it seemed as though he was pretty free.

“The morning session [on Monday], the 20-plus overs that we bowled, is probably as good as we’ve delivered. If that’s any indicator, [Starc] was a part of that, and as a collective that was relentless and really gave us an opportunity to control the game.”

Australia will regain the Border-Gavaskar trophy for the first time since 2014-15 if they win or draw the fifth Test.

Victory would also ensure that Australia book a spot in the World Test Championship final at Lord’s next year against South Africa, after the Proteas sealed their place in their own thriller against Pakistan on Sunday.

An India win in Sydney would level the series at 2-2 and allow the tourists to retain the trophy. The fifth Test will be the first time a Test series in Australia heads to the usual SCG finale with a trophy up for grabs since India toured here in 2003-04.