A back injury has left Michael Clarke doubtful for Friday's series-deciding fifth and final one-dayer against New Zealand.Team physiotherapist Alex Kountouris confirmed Clarke has soreness in his thoracic spine. The injury is different from the lower-back pain that has troubled Clarke in the past.It flared up during Australia's six-wicket victory over New Zealand in Adelaide on Tuesday when he opened the batting and bowled eight overs."It is not the lower back, where he usually has problems, but his upper back. He's sore and it's quite painful and I don't think he will be able to play," Kountouris said."But this type of thing doesn't hang around for long. (It) comes and goes quickly, so we expect him to get better fairly soon," he was quoted as saying by The Australian.Clarke, due to captain Australia in the Twenty20 match against New Zealand in Sydney on Sunday, will be given time until Friday morning to prove his fitness.If Clarke's condition lingers, it could force Ricky Ponting to play in the Twenty20 International despite he and Mike Hussey being rested by the selectors from that match.