It's just one more blow for the slumping Blue Bombers (6-9), who started the season on a winning streak but now find themselves with only a slim chance of making the playoffs.
They have three games left, two against the league-leading Calgary Stampeders (12-2), starting this Friday in Winnipeg. Their remaining home game after that is set for Oct. 25 against the B.C. Lions (8-7), now four points ahead of the Bombers in the CFL West race for a crossover playoff spot.
The Bombers announced Grigsby's release in a one-sentence statement Wednesday.
"I brought him into the office to explain what his role was going to be this week and he asked to be released," Bombers coach Mike O'Shea said later.
The team agreed to the release and O'Shea wouldn't say why a trade wasn't considered.
Grigsby leads the CFL with nine touchdowns and is fourth overall with 744 rushing yards, but he sat out last week's 41-9 loss to Edmonton.
Grigsby said his farewells on Twitter: "To all my fans I love all the support and they backed me into a corner! That I had know (sic) other choice! Only they can answer that!"
O'Shea tried to spark his team's faltering offence last week by giving Paris Cotton the start instead of Grigsby. Cotton had eight carries for 31 yards against the Eskimos, but O'Shea plans to give him another shot against the Stampeders.
"Last week in the first half we were so far behind that we weren't giving Paris a chance to get a look," O'Shea said. "He just wasn't going to get the touches so I needed to see him again."
The Bombers were down 37-0 at the half.
Cotton refused to answer any questions about Grigsby after Wednesday's practice, which was held behind closed gates because of police training in the stadium.
Cotton challenged for the starter's job in camp but was injured in a pre-season game and Grigsby claimed the spot. He took full advantage and led the CFL in rushing yards at one point when the Bombers were still on a roll.
Since then, their ground game has weakened and the losses have mounted.
Other players wished Grigsby well but slotback Nick Moore said he wouldn't quit over being asked to take a backup role, even though he wouldn't like it.
"If he (O'Shea) asked me to back up and I was healthy, I mean I would definitely be frustrated," said Moore, a high-profile free-agent signing for the Bombers. "That being said, I wouldn't, you know, ask to be released. I'm not saying anything about Nic, because I wouldn't do something like that, but I would just accept the role and do what I've got to do."
The Bombers, who are coming off one of their worst seasons ever, haven't won a game since Aug. 22. They finished with a 3-15 mark last year and cleaned house in the front office and coaching ranks.
This is O'Shea's first season as a head coach and general manager Kyle Walters' first full season in his position.
Winnipeg also has a rookie starting quarterback in Drew Willy, who has proved both durable and competent, although his effectiveness has been hampered by a league-leading 58 sacks allowed.
Willy is still second in the CFL in passing yards at 3,526, although it was uncertain Wednesday whether he'll be able to start against Calgary because of nagging problems with his throwing hand.
Winnipeg's offensive line has been blamed for his lack of protection and that certainly isn't new.
"I think since I've been here, everybody's kind of dumped on the O-line," said Veteran Steve Morley. "It sucks ... but all we can do is do what the coaches tell us."
The Bombers have had trouble recruiting Canadian starters like Morley, now in his sixth season with the team and ninth in the CFL.
"You can't make excuses, you know," he said. "We just have to be better. It's no one person's fault, it's a group effort and everyone in that room knows that."
Veteran defensive back Johnny Sears added the team can't give up with three games left.
"You go in that locker-room right now and we're gonna tell you we expect to win Saturday and that's the mindset we have to have," he said.