"It's going to be very tough going forward and I'm sure Clermont will be delighted they got us and not Harlequins," admitted O'Connell.
"It's going to be very tough for us to beat Clermont. They have power across the park and just look at their bench - any team in Europe would like to have their bench in their 1st XV.
"These young guys at Munster can give anyone a game, but it is going to be very tough."
Having played only seven games in the past 11 months due to knee and back injuries, O'Connell is still getting back to his best, but after his Man of the Match performance in the quarter-final in front of the three British & Irish Lions coaches, Warren Gatland, Graham Rowntree and Andy Farrell, he looks destined for a third tour this summer.
"I feel very much back in the mix after a game like that. There was a lot of doubt after last week - we felt we threw a bit of a dummy against Glasgow, which wasn't our intention, and we were a bit down in the week," said O'Connell.
"To be back involved with it is great. I've a little bit to go with my match fitness, but I'm delighted to be back playing and to have come through another match injury free."