Elsewhere, Melbourne skipper Max Gawn will miss two to three weeks with an ankle injury.
Lewis, 25, suffered the injury after landing awkwardly following a heavy collision with Geelong's Jake Kolodjashnij in the final quarter of Saturday's loss at GMHBA Stadium.
The luckless key forward, playing his first game since round three, left the field with blood streaming from his head and looked to be holding his knee."Our thoughts are all with Mitch at this point in time," football boss Rob McCartney said in a statement."He has worked so hard to get his body right and return to AFL level and this injury was a cruel blow."While this will spell an end to his season, we are confident that with expert guidance of our medical team and the strength of Mitch's character that he will work his way through the immediate challenges that lie ahead."Only being 25 years of age, I'm extremely confident that following his rehab Mitch has a bright future ahead in the brown and gold and is an integral part of what we are building at our football club."
Gawn is set to miss games against Essendon, Fremantle and potentially GWS.
Max Gawn (right) injured his ankle during Melbourne's easy win over the West Coast Eagles. (James Ross/AAP PHOTOS)
Gawn hurt his right ankle late in the second quarter of Sunday's 54-point win over West Coast and was substituted out of the game during the final quarter.
The injury occurred when Gawn went to kick the ball out of the centre and his foot instead cannoned into the body of West Coast's Harley Reid.
Gawn, 32, was on crutches at AAMI Park on Monday and said he was "very sore" before the extent of his injury was revealed.
"Scans have today shown that Max has sustained a chip to the base of his right fibula," Demons football boss Alan Richardson said.
"While this is disappointing news for Max, fortunately he won't require surgery.
"Max is the ultimate professional and leader and we know he'll attack his rehab and continue to play a pivotal role in our group over the next few weeks, albeit from the sidelines."
West Coast's star defender Jeremy McGovern was taken to hospital with suspected rib and lung damage following a final-quarter collision.
Jeremy McGovern has been discharged but remains in Melbourne. (James Ross/AAP PHOTOS)
McGovern was discharged from hospital on Sunday night, but remains in Melbourne with a club staff member.
West Coast say they will give an update on the extent of McGovern's injuries when they have more details.
Skipper Oscar Allen, who kicked four goals, will be further assessed for an ankle injury.
Western Bulldogs defender Liam Jones will miss two to four weeks with a knee injury, while Jason Johannisen (calf) is out for five to seven weeks.
Recruit James Harmes (hamstring) could be sidelined for a month, while concussed pair Aaron Naughton and James O'Donnell will miss at least one week.
Cody Weightman (groin) will have to prove his fitness to return, as will Alex Keath (hamstring).
