Sean Cronin was placed on stand-by while Mike Sherry was flown to New Zealand as emergency cover after hooker Best injured his shoulder during the 36-6 defeat of Italy that secured Ireland top spot in Pool C.
Cronin has been stood down to the bench though after Best recovered in time to face Wales and defence coach Les Kiss insists the 29-year-old is raring to go.
"Rory's come up great and is fine to start the game," said Kiss, who saw first-choice hooker Jerry Flannery sent home with a calf injury last month.
"We obviously had some precautions in place ready to cover Rory if he failed to pull through, but he's in tip-top shapeso full credit to him and the medics.
"This game is absolutely massive for both teams. We spoke earlier about what it would mean to people back home if we won.
"We want to reach a place we've never been before. We've played each other so much that it's fair to say it's 50-50.
"There will be critical pressure moments and it will be a 60-minutes plus game. The last 20 minutes will sort it out."
Ireland's other main selection dilemma came in the half-backs where the experienced Ronan O'Gara and rookie scrum-half Conor Murray were preferred to Leinsterpair Jonathan Sexton and Eoin Reddan.
"I know it's unconventional to use different half-backs at different times," said coach Declan Kidney.
"When they've been on the pitch they've brought their own things to it.
"For example, Jonathan was hugely strong for us in defence in the last 10 minutes against Italy. Eoin robbed two great balls for us in the last 10-15 minutes.
"Leaving them out isn't easy, but Conor and Ronan have gone quite well for us."