It's a massive blow as he will miss the remaining Heineken Cup pool fixtures against Neath/Swansea Ospreys and NEC Harlequins at Thomond Park and Twickenham.
Coach Alan Gaffney will be comforted by the fact that Paul Burke is ready to slot into the team, and the ex-Irish international now looks an inspired signing during the summer.
Gaffney said: "It's a bad blow to lose a player of Ronan's calibre. He has been a huge influence over the past number of seasons, but it could have been worse. We have quality back up and Paul is a player with huge experience of top class rugby, including the Heineken Cup. It's extremely comforting that we can call on a player of his stature."
Neath-Swansea Ospreys assistant coach Sean Holley agreed that O'Gara will be a huge loss to Munster.
However he is adamant that Paul Burke is a more than a capable replacement.
"We got a message that he had been replaced yesterday and obviously we were aware that something serious had happened.
"We recognise that Munster will suffer a huge blow without Ronan especially as the games have been so tight between us in both the Celtic League and in the Heineken Cup.
"Ronan has been a major player in the last two wins over the Ospreys. Naturally we will be slightly pleased if he doesn't play. But we wouldn't underestimate Munster. They are an experienced outfit and their pack is beginning to peak."
O'Gara landed five penalties when Munster defeated the Ospreys 20-18 on October 31st at the Gnoll. Prior to that, he had kicked one penalty and one conversion in the 15-9 victory over NEC Harlequins in the Pool Four opener.
"I was in university with Paul Burke and we are good friends. I played with him and against him and he is more than creditable replacement. Paul will revel in it. He has played rugby in England, Wales and Ireland at the top level.
"He is very comfortable on the ball out of hand and for goal. In a way I think that Munster may benefit in some areas from the change."
Holley's side are well aware of what awaits at Thomond Park on Saturday.
"We are expecting a big reception. We've had some cracking games as preparation with matches that have been physical and tight. We are well aware of what is to come."
There was better news on other wounded heroes. Anthony Foley and Rob Henderson, both replaced in Saturday night's clash, will be fit to face the Ospreys. So too will John Hayes, who, due to illness, was forced to miss Saturday's game.
There was a huge sense of occasion before kick off at Musgrave Park with a record crowd of 8,000 witnessing a brief fireworks display to greet the New Year followed by a period of silence in memory of the victims of the tsunami disaster in South East Asia.
Munster emerged from the scrap with four priceless points that maintains their vibrant challenge to leaders Ospreys.
A late try by Eric Miller and conversion from Dave Holwell allowed Leinster to take a bonus point, but this was a more comprehensive victory than the score line suggests.
Munster won't thank referee Alan Lewis for a succession of questionable decisions in the latter stages of the game but by then they had already secured the win after scoring two typical third quarter tries.
Leinster did look the more potent force behind the scrum - no surprise given the players at their disposal - but Munster did exactly what was expected of them up front and denied the visitors quality possessions.
Leinster had nobody to match the form of Paul O'Connell, Anthony Foley, Denis Leamy and particularly Alan Quinlan, who produced his most consistent display for the province.
The Munster defence was mean and Leinster's only real threat came from broken play. Brian O'Driscoll and Gordon D'Arcy had to work with limited possession in a carefully controlled defensive zone.
Leinster coach Declan Kidney conceded it was a worthy Munster victory and his counterpart Gaffney was obviously chuffed.
"It was the first time since the Celtic League final a few years back that both sides were at full strength, the first opportunity to really assess both. In those circumstances, I'm very happy with the win."
Munster's performance in the third quarter was the key to victory after they turned around to face a strong wind with a 9-3 half time advantage
Hooker Frankie Sheahan got the second of Munster's tries to push the lead out to 19-3 and, without wanting to take credit for the score, recognised the importance and timing of it.
Settling personal scores in an environment where places on the Irish team were clearly up for grabs was never an issue, according to Sheahan.
"It was obviously tempting for a number of players to look to their own personal performances, to get involved in personal battles but it never happened."
The line out was a huge plus for Munster. O'Connell and co managed to turn over four of Leinster's throws and remained untroubled in the scrums, where Emmet Byrne appeared to struggle for Leinster.
O'Gara kicked two penalties before his departure and Burke added another to reply to David Holwell's effort.
But Munster stormed their way to victory after Burke missed a penalty and Horgan was denied a try. Marcus Horan got the touch after a rolling maul in the 12th minute of the second half and Sheahan finished Leinster off with the second midway through the half. Miller's fine 76th minute try after Holwell had kicked his second penalty gave Leinster hope. But they were never going to stop a determined Munster.
: C. Cullen, S. Payne, M. Mullins, R. Henderson, A. Horgan, R. O'Gara, P. Stringer, M. Horan, F. Sheahan, G. McIlwham, D. O'Callaghan, P. O'Connell, A. Quinlan, A. Foley (captain), D. Leamy. Replacements: P. Burke for O'Gara (injured, 38), J. Williams for Foley (60), M. Lawlor for Henderson (injured, 68).
: G. Dempsey, S. Horgan, B. O'Driscoll, G. D'Arcy, D. Hickie, D. Holwell, G. Easterby, Re. Corrigan (captain), S. Byrne, E. Byrne, L. Cullen, B. Gissing, E. Miller, V. Costello, S. Jennings. Replacements: R. Nebbett for E. Byrne (63), C. Potts for Gissing (83).
: A. Lewis (IRFU).