Heckenberg has World Cup experience with Scotland
Scotland have lost Super League players Daniel Heckenberg and Rhys Lovegrove from their World Cup squad.
Harlequins prop Heckenberg, who played in all three of Scotland's matches in the 2000 World Cup, and Hull KR centre Lovegrove are both injured.
The Scots have also dropped Wigan winger Mark Calderwood after his call-up to England's train-on squad.
"We only want players who are committed to Scotland so he's out of our squad," said manager Richard Thewlis.
Coach Steve McCormack's initial 40-strong squad was already hit by the loss of Workington back-rower Iain Marsh, with a broken arm, while Richard Fletcher (Widnes), Spencer Miller (Whitehaven) and Graeme Horne (Hull) all pulled out for personal reasons.
McCormack will name his final 24-man squad on 8 October.