In the replies to this post on X Dr. Jesse Morse an expert on knee, hip, and shoulder injuries chimed in. He believes that Auston Matthews could be dealing with a significant MCL or ACL injury.
With all of this in mind, this is just speculation based on watching a few clips of a potential injury. As of now, the Maple Leafs continue to say Auston Matthews is out due to sickness. However, if his knee is in fact compromised it could mean a frustrating end to Auston Matthews' season and a delay to his debut again next year.