Glenn McGrath receives treatment to his injured ankle from physio Errol Alcott. McGrath will miss the third Test starting on Thursday, as might Brett Lee © Getty Images

Brett Lee has given Australia a fresh fitness worry ahead of the third Test on Thursday, following his admission into a Birmingham hospital late on Monday afternoon after experiencing swelling and discomfort in his left knee.

Australia's physiotherapist, Errol Alcott, told reporters at the team hotel that doctors were confident Lee would respond to treatment for the infection, but he remained in hospital overnight, and that he would not travel with the team to Manchester this afternoon. Lee is presently on an intravenous drip and Alcott described his condition as "comfortable", adding that the injury to Lee's left knee may have been related to a cut he sustained while fielding during last month's first Test at Lord's.

Asked about Lee's chances of playing in the third Test, Alcott added: "He's 50-50 but that's what I always say." The injury first came to light before Lee batted on Sunday when he complained of soreness. He went onto score an unbeaten 43 that almost guided Australia to a stunning victory in the second Test at Edgbaston, a match England won by two runs to level the series at 1-1.

Australia, the winners of the last eight Ashes series, are already facing up to the absence of their fast-bowling spearhead, Glenn McGrath, who sustained an ankle injury in the warm-up before the start of play last Thursday. "Glenn is proceeding as well as expected," added Alcott. "He had a scan today and that showed extensive ligament damage, but we knew that anyway."

If Lee was to be ruled out, it would be a huge blow to Ricky Ponting's team, who do have the uncapped paceman, Shaun Tait, in their squad as cover. Prior to Lee's injury, Australia selectors were already discussing the possibility of calling up the allrounder, Shane Watson, who is playing for Hampshire.