Grigg has missed the last two games with a knee injury and will sit out Saturday's Sky Bet Championship clash with Aston Villa, as well as Northern Ireland's World Cup qualifier against Norway, as his condition is assessed.
Interim manager Graham Barrow has already given up hope of defender Donervon Daniels playing again before the end of a campaign as he attempts to fight his way back from his own knee problem.
However, Barrow will have Sam Morsy available after he completed a two-match ban and Shaun MacDonald (also knee) was used as a late substitute in last weekend's 1-0 home defeat by Bristol City, while fellow midfielder Max Power continues to battle a hamstring strain.
Henri Lansbury could return for Aston Villa after he was expected to train on Friday ahead of the game.
Striker Scott Hogan has also trained this week after suffering a knee injury at Newcastle last month and will be assessed.
Jack Grealish (knee) has trained having missed the last two games, a defeat to Huddersfield and victory over Sheffield Wednesday.
But Nathan Baker (knee) is out of the trip along with Andre Green (hamstring) and Rushian Hepburn-Murphy (knee), who is due to return to training next week.