Ireland hooker Sheehan ruled out of second Springboks Test
"Dan suffered a serious knee injury during the first Test and may not play again this year," he said on condition of anonymity as he is not an official spokesman.
Sheehan sustained the injury after tackling Springboks lock Eben Etzebeth during the first half of a 27-20 loss in Pretoria last Saturday.
He was replaced at half-time by Leinster teammate Ronan Kelleher, who is likely to start in Durban with Rob Herring providing cover on the bench.
Scrum-half Craig Casey, preferred to vastly experienced Munster teammate Conor Murray in the Pretoria starting line-up, could also miss the final match of the series.
Casey was taken from the field on a stretcher suffering from concussion after being tackled by Springbok local RG Snyman.
Murray would be the obvious replacement if Casey is ruled out with Caolin Blade promoted to the bench.
There are a number of other injury concerns for head coach Andy Farrell ahead of the announcement on Thursday of the second Test team.
Centre Robbie Henshaw is going through return-to-play protocols after suffering a head injury at Loftus Versfeld.
Full-back Jamie Osborne (groin), centre Bundee Aki (shoulder), winger James Lowe (thigh) and prop Andrew Porter (hand) are others facing fitness tests.
A win for Ireland in the Indian Ocean port city would tie the series, and end a run of four series losses since their first multi-match tour of South Africa in 1981.
Record four-time world champions South Africa will name their second Test matchday 23 on Tuesday for a match that will be watched by a sell-out 52,000 crowd at Kings Park stadium.