Both McCulloch and Farrelly have been confined to the bench in recent weeks as Latics closed in on a top-six spot.
But now they have the opportunity to play a major part in pushing the team closer to glory.
Scotsman McCulloch will step in for Roberts up front at the City Ground, and again when West Ham visit the JJB a week on Sunday – Roberts got a two-match ban as the red he got at Selhurst Park was his second of the season.
Jarrett's ban is only for the Forest clash, giving Farrelly just 90 minutes to press his claim for a regular start.
Manager Paul Jewell is disappointed to be hit by the double suspension but believes he has the necessary cover.
He said: "We could have done without the two lads being banned because we haven't got the biggest of squads.
"But we have people who can come in and give a good account of themselves.
"If that was me and there was a chance of getting into the team and impressing the manager at this important stage of the season, I would be looking to grab it with both hands. I know who will be coming into the team although things can still change before the weekend."
McCulloch's last start was at Watford last month, a game in which he played on the left-wing.
He stood in for Roberts in attack when the striker served a three-match ban in March.
Farrelly has made just one start since joining Latics from Bolton on a short term contract. That was at Bradford when he was substituted at half-time because of a shortage of match fitness.
The former Republic of Ireland international has since made three substitute appearances as he tries to earn a longer contract at the JJB.
The team changes will also leave a couple of vacancies on the bench to be filled.
Striker Neil Roberts, who hasn't appeared in the 16 for the last four games, will be pressing for a recall.
So to will be Mike Flynn, Steve McMillan and Tony Dinning, who has been out of favour for a large chunk of the season.
* Saturday's game is now all-ticket for Latics fans.
The home areas have sold out and police are anxious that Forest fans don't gain access to the away end through pay-on-the-day turnstiles. Latics have so far sold 1,400 of their 2,300 allocation, and the JJB ticket office will remain open until Saturday lunchtime.
* FA Cup Final referee Jeff Winter will take charge of Saturday's game, one of his last before retiring in the summer.
Source: Wigan Evening Post.
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