Ireland's Jared Payne© Michael Steele/Getty Images

Jared Payne must prove his fitness on Thursday to be eligible for selection for Ireland's pivotal Rugby World Cup clash with Italy on Sunday.

The Ulster centre sat out training on Wednesday with a bruised foot and now faces a race against time to be ready for the crucial Pool D encounter.

Rob Kearney is another doubt to face the Azzurri at London's Olympic Stadium as he continues to recover from his gluteal strain. Simon Zebo has rejoined the Ireland camp after heading home for his grandfather's funeral, and would slot in for Kearney at full-back if required.

"If guys have had niggles they get through this afternoon, they will be assessed in the morning and then it's selection from there," said Ireland's scrum coach Greg Feek. "So Rob ran around and a few guys looked pretty good so fingers crossed for the rest of the week. 

"Jared Payne didn't train today. He's just got a bit of a bruise on his foot, he didn't train, so we'll see how he goes for the rest of the week. 

"Rob trained so now it's just a matter of how he goes from that. Normally you like to give them a run and then assess that, to see if they wake up with anything, if it's sore or whatever. So from here we'll be able to assess that." 

Should New Zealand-born Payne fail to recover from his foot problem in time to face Italy, head coach Joe Schmidt could opt to slot Munster's Keith Earls into the outside centre berth, with Luke Fitzgerald and Dave Kearney likely to start on the wings.

Earls featured at 13 in Ireland's 35-21 World Cup warm-up victory over Wales in Cardiff and stood up well, albeit against a mix-and-match home line-up.

Feek admits Ireland must select a genuine first XV against Italy, where victory will secure their place in the quarter-finals. 

Cian Healy has been pushing hard after recovering from neck surgery just in time to make the World Cup, with Feek relishing his battle for the loosehead role with Leinster club-mate Jack McGrath.

"I think the best team to beat Italy has to take the field, yes," said Feek. "And I think we had a good hit-out against Canada but now we've got to get that quarter-final and we've got to get out of the pool. So we've got to move on now. 

"That's not saying the team that plays this week will play next week, but the team that plays this week will be the one we think that can best combat Italy. 

"That competition between Cian and Jack, that's always been there -- both of them respect each other for what they do and I think they've got different strengths.

"Jack's had some consistent form of playing really well, then Cian's come back and he's done really, really well to get back. And I think in some ways part of him is really, really grateful to be playing here right now, and I can see the fires starting to burn. 

"He wants more, and he wants that jersey back already. So it's exciting to have those two going hammer and tongs at each other."