Republic of Ireland skipper Roy Keane flew home to Manchester yesterday after conceding he would be unable to play in the second leg of the World Cup qualification play-off on Thursday in Tehran.

The 30-year-old midfielder had admitted on Friday his continued discomfort with the knee injury that had kept him out of Manchester United's last five games but was "taking it one day at a time" and waiting to see whether there would be any reaction following Saturday's first leg game against Iran.

Immediately after the match the Corkman said he felt he would be okay to travel out with the rest of the squad early today, although he gave some hint of potential difficulties by adding he would first consult with the physiotherapist at Old Trafford and with Alex Ferguson. But yesterday morning the soreness in his left knee seemed to have worsened a good deal and after a meeting with Mick McCarthy, it was announced he would travel back to England.

"His knee stiffened up overnight," McCarthy said yesterday, "so, in the circumstances, it would be expecting too much of him to play two games in five days.

"Obviously we'll miss him," he added, "because he's one of the best players in the world. I thought it was fantastic what he did on Saturday, but now we've got to go in without him and there's not much we can do about that. It is all about the squad now and you only have to look at our bench to see the quality we've got."

While McCarthy gets on with the business of preparing for Thursday's game without Keane, his hope now will be that his most gifted player can resolve the problem without needing surgery that would, in the event of Ireland actually qualifying for next summer's World Cup finals, rule Keane out of a good portion of the team's preparations.

Keane's withdrawal means Mark Kinsella will come into McCarthy's team on Thursday, with the Charlton midfielder set to partner Matt Holland for only the second time in this qualification campaign. The pair previously linked up in June against Estonia, a trip Keane missed due to suspension, and the partnership appeared to work well on a night when Ireland emerged as 2-0 winners.

The absence of Keane is by no means McCarthy's only worry ahead of Thursday's game, however. Steve Staunton and Niall Quinn both picked up serious injuries on Saturday and while both are expected to travel this morning, neither is certain to be ready to play at the Azadi stadium.

Quinn suffered a recurrence of his back problem early in Saturday's victory but was left on by McCarthy in case his height was required to defend against Iranian set-pieces during the second half.

After the game he said that unless he staged a major recovery he would be unfit to play on Thursday. "I am in agony at the moment," Quinn conceded. "I have had a back problem for a long time, as everyone knows, and this knock hasn't helped. I am sore and stiff at the moment and I need to get treatment on it. But I will fly out with the squad. We have four days out there before the game and that time should help me to recover before kick-off."

If Quinn isn't fit to play then David Connolly or Clinton Morrison is the likely replacement, but McCarthy's remarks regarding the big man's value as an additional defender suggests he will be desperately anxious to have the 35- year-old available for a game in which the Irish can expect to come under a good deal more pressure.

Staunton, meanwhile, limped out of Saturday's game with what initially appeared to be a hamstring problem but which, it subsequently emerged, also turned out to involve the player's back.

"I think it's more my back than my hamstring," he remarked. "I will need treatment but I am desperate to play in this game and get us to the World Cup finals. I can make it for Thursday night."

Though the 32-year-old has been one of the team's most impressive performers during the latter part of this campaign, McCarthy's defensive options are improved by the return to full fitness of Kenny Cunningham, who came on for Staunton on Saturday, while Richard Dunne and Andy O'Brien are also among the travelling party this morning.