Ulster's Nathan Doak and Connacht's Dave Heffernan have both been called up to the Ireland squad for Saturday's second Test with South Africa in Durban.
The IRFU confirmed the pair have linked up with the squad at their team base in uMhlanga, north of Durban, ahead of Saturday's meeting with the Springboks at King's Park.
Sheehan was replaced at half time in last weekend's 27-20 defeat to the Springboks in Pretoria, having suffered the injury to his left knee after tackling second row Eben Etzebeth.
While it was initially thought the player had suffered a more minor knee injury, it's now suspected he's suffered a ruptured anterior cruciate ligament, which could see him ruled out for up to nine months, and leave him unlikely to feature again before the end of the 2025 Six Nations.
It would also damage his chances of being picked on next year's British and Irish Lions tour of South Africa, for which he had been a strong favourite to be a Test starter.
Heffernan, who won the last of his seven caps against New Zealand in July 2022, will join Rónan Kelleher and Rob Herring as the hooker options for head coach Andy Farrell.
Doak's call-up is his first time involved with the senior squad since November 2022, while he also featured for Emerging Ireland in South Africa that same season.
The uncapped 22-year-old scrum-half is drafted in to support Conor Murray and Caolin Blade as the potential options behind the scrum.
Casey needed lengthy treatment on the pitch at Loftus Versfeld on Saturday after hitting his head following a tackle by former Munster teammate RG Snyman, before being carted from the pitch with a concussion.
The IRFU have not given an update on the other injury doubts in the Irish squad.
Andrew Porter is expected to be cleared to play on Saturday, despite suffering a nasty cut to his hand during Saturday's game, while Robbie Henshaw, Bundee Aki, James Lowe and Jamie Osborne are all believed to be carrying knocks.
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