Ex-Aberdeen defender Diamond's Hearts move in doubt
The 26-year old is currently a free agent after his Dons contract expired but had agreed a deal to join Hearts this summer.
However, an ankle injury could jeopardise the move with Hearts boss Jim Jefferies waiting to hear more from his medical team.
"We won't know the full position until we sit down for discussions with the medical people," Jefferies told the Daily Record.
"Once we're in possession of all the information we'll be better placed to know if we can take the deal forward."