Toews said the following regarding his status in a statement:
"This offseason, I've been experiencing symptoms that have left me feeling drained and lethargic. I am working with doctors so I can better understand my condition.
"Until I can get my health back to a place where I feel I can perform at an elite level and help my team, I will not be joining the Blackhawks for training camp. I do not have a timetable for when I will rejoin the team. I am extremely disappointed, but it wouldn't be fair to myself or my teammates to attempt to play in my current condition."
The 32-year-old has been fairly durable in recent years, appearing in at least 70 games in each of the last seven seasons for one of the league's marquee franchises.
He played a crucial part in Chicago's three Stanley Cup titles since 2010. He won the 2009-10 Conn Smythe and 2012-13 Selke Trophy and scored at least 18 goals every season of his 13-year career.
Last season, Toews netted a career-low 18 goals in 70 games, but he was still highly productive with 60 points, which was his second-best total since 2014-15.
While the Blackhawks rely on his veteran leadership on the frontline, they do have plenty of weapons to carry the offensive load in his absence should he not be ready for the start of the 2021 season in January.
Patrick Kane is one of the most electrifying players in the league, Dylan Strome can occupy the center spot, and Alex DeBrincat and Dominik Kubalik are likely to help pick up the slack without Toews.
Chicago could use Toews back and healthy before the stretch run in the race to the Stanley Cup playoffs, but it does have enough pieces to remain afloat if he misses significant time.