Essendon have suffered another major blow a week out from the start of the AFL season, with a calf strain further delaying Joe Daniher's comeback.
Daniher reported tightness after Wednesday's training and a scan on Thursday confirmed what he called "a little nick".
The Bombers said the 2017 All-Australian would be out of action for four to five weeks.
Daniher only managed seven games last season before he was sidelined with osteitis pubis.
He is vital to Essendon's hopes of returning to the top eight and winning their first finals match since 2004.
The Bombers also are without key position player Cale Hooker for the round-one match against GWS after he also suffered a calf injury earlier this month.
Like Daniher, Hooker will be out of action for around four weeks.
Daniher played in Essendon's two official pre-season games and had been upbeat about being ready for round one.
"The body is feeling really good and I was pulling up really well," he said.
"This is a little bit annoying, that it's come now - I would have loved to have been out there in round one with my teammates and playing for the football club."
Daniher said he was yet to meet with club medical staff and work out a timeframe for his return from this latest injury.
"I will be back playing as soon as it's physically possible," he said.
"I will be convincing them that I'm right to go - but I won't be making the same mistake twice."
While it is a significant blow, Daniher tried to put a positive spin on his injury.
"It's just a bad bit of luck and that happens, but the opportunity now is to get it right, take a little bit longer to fine-tune what I've been working on (with) my game," he said.
"Hopefully I will come back in and be able to perform, rather than ... just play.
"There's never a good time to get injured, but I have a fantastic opportunity now to take a little bit longer, work a little harder on those little things I want to fine-tune in my game and hopefully come back a better player."
Daniher, 25, reached 100 AFL games last season.
He wants to work on his leadership and be able to play better for longer during matches.