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."Most importantly, our focus at present is to support Mitch and wrap our arms around him."
Elsewhere, Melbourne skipper Max Gawn predicts he will miss Saturday night's crunch clash with Essendon due to a painful ankle injury.
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 rising star Harley Reid.
Gawn, 32, was on crutches at AAMI Park on Monday, wearing a moon boot on his right foot, and has undergone scans.
"It's pretty sore, I must admit. I woke up very sore. Obviously played the second half in a bit of distress," Gawn told reporters.
"At this stage, I'm thinking that (I will miss), just because of how sore it is, but I've seen some stuff that the human body can do and it can certainly turn around pretty quickly.
"I'm always looking quite optimistic at these things. I don't exactly know what it really is yet, so we're waiting for a bit of information. I'm sure we'll get it to you this afternoon.
"I'd say it's more of a short-term thing - I'm hoping, but it is pretty sore."
West Coast were also left with injury concerns.
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.