London - Australia captain Michael Clarke could miss the fifth one-day international against England on Monday after suffering a recurrence of his chronic back problem.
The Australians had already planned to recall opener Phil Hughes in place of Shaun Marsh, who is ruled out after hurting his hamstring in the fourth one-dayer, and they may now need to replace Clarke as well.
England levelled the series at 1-1 in Cardiff on Saturday, setting up the prospect of an exciting finale, and Clarke is still hoping to play.
But coach Darren Lehmann insisted Clarke would only be allowed to play if he is fully fit for the match in Southampton.
"It's a big game. But if he's not right, we won't risk him," Lehmann said on Sunday.
"It's an important series, but we've got another one-day series against India and an Ashes series coming up."
Lehmann confirmed Hughes, who last played for Australia in the second Test at Lord's earlier in the season, is almost certain to come into the line-up given his side's injury problems.
"We'll look at the pitch but I think that's probably the way we'd go," Lehmann added.
"It gives him an opportunity. He's playing really well and working on his game, and that's all we can ask.
"He's ready to go. We hope he gets his chance and away he goes."
Should Clarke be unavailable, Australia may call all-rounder Glenn Maxwell into their squad for the first time in the series, with leg-spinner Fawad Ahmed another option.
Australia have endured a disappointing tour, an early exit in the Champions Trophy followed by a 3-0 Ashes defeat and then a 1-1 draw in two Twenty20s.
But Lehmann hopes they can finish the tour on a high note by winning the 50-over series and he said: "It gets down to a grand final set-up now - which is great.
"That's what we want the players put under most of the time, although you would have loved to close out the series (in Cardiff).
"We're certainly playing the right brand of cricket. We just made a couple of elementary mistakes.
"Apart from that, we've pretty much played how we want to play.
"Now the case is we want to finish on a high and get moving on."