Dr Claire Evans, a pathologist from the Royal Hospital for Sick Children at Yorkhill, Glasgow, conducted the post-mortem examination on the 15-month-old.
She did the medical examination after Declan was found dead in his rubbish-strewn cot at his home in Paisley, Renfrewshire, in March 2010.
Declan's mother, Kimberley Hainey, was jailed for murder but was released last April after appeal judges quashed her conviction.
A Fatal Accident Inquiry into his death is due to start next week, and yesterday at a hearing ahead of the probe, it emerged that Dr Evans was not going to be called to give evidence.
But following a request by Edward Targowski, Kimberley Hainey's QC, Dr Evans, who is a consultant paediatric and perinatal pathologist, will have to attend Paisley Sheriff Court to take the stand.
Last month it emerged that a drug worker who had several meetings with Ms Hainey around the time of Declan's death would have no participation at all in the case due to ill health.
Renfrewshire Council, two doctors, five health visitors and seven social workers will also be represented at the inquiry, which is due to start on Monday.