The on-loan Sunderland striker picked up a hamstring injury in Tuesday’s 1-0 win at AFC Wimbledon and scans have shown severe damage.
Boss Paul Warne does not think Grigg will be fit until some time in April, meaning he will definitely not be available for the short trip across the M1 on Sunday lunchtime.
“Griggy coming off injured is a big blow, I think he is the glue to the team really,” Warne said.
"His link-up play is really good and he works well with Michael Smith.
"Unfortunately, he's going to be out for a long period of time, so that isn't good news.
"I don't really like to put a timeframe on Griggy's because the physios told me, but we think he might be about eight weeks.
"His is a severe hammy. There was obviously nobody near him when he did it.
“He ran and put the cross in for a chance that Smudge should definitely have taken.
"It's a massive blow. Griggy has got back to looking really sharp and his offside goal was very harsh as well.
"He'll be out for a period. As I always say, it's 'next man up' but I do say that with a heavy heart because I do think we're going to miss him."
Grigg’s absence means Warne has a decision to make as to who partners Michael Smith in attack.
Josh Kayode scored the winner at Wimbledon having come on for Grigg so he is contention to start, but Warne might opt to recall Freddie Ladapo, who scored a memorable injury-time winner at Hillsborough last season.
Smith is a definite starter having been named as January’s League One player of the month.
The striker, who won October’s award, scored five goals last month, said: “It's really nice to have picked up the award twice this year from a personal point of view, but as I said last time, this is an award for all of the team's efforts.
"I might have got some of the goals but I wouldn't be able to do that without the platform that everyone else provides and you only need to look at how many clean sheets we're keeping to see that this is an effort from across the pitch.
"As grateful as I am to be Player of the Month, I'd trade the personal accolades for us to be promoted come the end of the season. That is the overall objective and that is something you can only achieve as a team and not by yourself."
Meanwhile, the Football Association is investigating the crowd trouble during last week’s win over Accrington.
Two fans invaded the pitch to disrupt a penalty to Accrington, with one of them barging into Harry Pell.
Both men have been identified and handed lifetime bans.