O'Connell hasn't played since Munster defeated Edinburgh in the Heineken Cup back in October after his long-standing back injury flared up and is considering an operation in an effort to clear up the problem.
A decision on whether or not O'Connell will go under the knife is expected to be made this week.
Meanwhile, it has also emerged that Ronan O'Gara was a serious doubt for Munster right up until the kick-off against Saracens on Saturday after injuring his back in a gym session earlier in the week.
The out-half took no part in training after Wednesday. Indeed, so concerned were Munster regarding his availability that JJ Hanrahan was, at the last minute, withdrawn from the Munster 'A' team that played at Rotherham in the British and Irish Cup earlier in the day.
If O'Gara had not been able to play, Ian Keatley would have started, with Hanrahan on the bench.
Simon Zebo and Dave O'Callaghan shipped knocks during Munster's win, but coach Rob Penney believes they will both be available for selection for the return game at Vicarage Road on Sunday afternoon.
Munster will face a huge task this weekend on the evidence of Saturday's action, especially given the fact that Owen Farrell will surely have a better return from the boot than the 43pc conversion rate he reached at Thomond Park.
Ronan O'Gara landed five penalties for Munster while Saracens centre Farrell, who had an indifferent night with his goal-kicking, secured a losing bonus point for the visitors with his third penalty late on.
"They are all must-win games to be fair. We are in a tough pool. But Sunday's game is a must-win," said Penney afterwards.