The Irish have been left counting the cost of Sunday's 24-9 win over France, which secured them top spot in Pool D, after losing three key players to injury during the clash at the Millennium Stadium.
O'Connell's injury would appear to be the most serious, the second rower having left the field of play on a stretcher and Schmidt admits he could miss the remainder of the World Cup as a result.
The 35-year-old is set to retire from international rugby after the tournament has finished and Schmidt fears he may have played his last game for the men in green.
"It's probably Paul that worries me most of the three," observed Schmidt.
"It doesn't look great with Paul. It looks like an upper hamstring but we will probably have to wait until Monday to give it a scan."
Fly-half Jonny Sexton is also set to undergo a scan on Monday to discover the extent of the groin injury he suffered against France, with Schmidt keeping his fingers crossed he will be fit for the quarter-final clash with Argentina.
"With Johnny, it looks like an adductor to me, but it's not the learned eye of a medic. So we are probably going to get him scanned tomorrow," he added.
"He's already talking about next week. I'm not sure whether that means he'll be playing. He's never short of advice."
The Ireland coach was more upbeat on the prospect of Peter O'Mahony being fit to face the Pumas though with his knee injury not as serious as first feared.