Sexton limped off in the 25th minute of the pool victory over France last weekend and initially it looked like his injury would be serious enough to force him out of the tournament, along with lock Paul O'Connell and flanker Peter O'Mahony.
Scans revealed on Monday that the injury was not as bad as first feared, however, and the influential playmaker was back on the training pitch in Cardiff on Wednesday.
"Johnny did about three and a half kilometres running, and that’s a positive, so we’re quietly confident (with him) doing that and we’ll see how he goes again on Friday," Feek told reporters.
Now O'Connell and O'Mahony have been replaced in the squad, Ireland's only other injury concern is outside back Keith Earls.
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"He’s just a bit battered," Feek added. "It's not bad, it’s nothing too major. At this stage we want to do the best we can to make sure we can get everyone fit and available."
Although coach Joe Schmidt usually insists players must be available to train in midweek if they are to play at the weekend, Feek said there might be more latitude this week.
"It's do-or-die kind of stuff, so there might be some allowances around some things," Feek said.
Another loss to the Irish is openside flanker Sean O'Brien, who was handed a week's ban on Tuesday after being cited for hitting French lock Pascal Pape in last weekend's match.
Ireland have 48 hours to appeal the ban and Feek said they might consider that course of action.
"We’ll wait for the written (judgement) to come through before we decide whether we will appeal," he said.
Despite having lost O'Connell and O'Mahony, and with at least one of O'Brien and Sexton looking likely to miss Sunday's match, Feek was confident the back-up players would step up as Ireland bid to reach their first World Cup semi-final.
"We’ve got guys sitting there raring to go; some guys haven’t played much," he said.
"You’ve heard the mantra about the collective and that’s something this team prides itself on."
(Editing by David Goodman)
