Lewis requires a knee reconstruction after tearing the anterior cruciate ligament in his left knee during last week's loss to Geelong.
It was the 25-year-old's first game back in more than three months after he battled cartilage issues in the same joint.
Lewis has a long history of knee problems and it was hoped his return would boost the Hawks' finals chances this season.
But he will now be sidelined until well into the 2025 campaign.
"Amazingly, he's been into the club," Mitchell said on Thursday.
"He doesn't need to be doing anything, he's got a brace on his knee ... but he's still going to line meetings and having an impact and having his input into how we need to play to beat Freo (on Saturday).
"It's important for him that everyone sees that and he's still a great person to have around the club. Everyone's pretty proud of how he's handled it so far.
"There will be some down times and he's going to have the surgery over the next couple of weeks, so there are some challenges ahead, but at the moment it's onward and upward."
Mitchell is weighing up how best to replace Lewis as the finals race heats up, with Hawthorn (8-8) hosting third-placed Fremantle (10-5-1) in Launceston on Saturday.
Calsher Dear, Max Ramsden and mid-season recruit Jasper Scaife are all in the mix as tall options to play alongside main target Mabior Chol and veteran forward Jack Gunston.
"He's one of those guys who was always in our planning and now to have him out of that for an extended period has caused us a little bit of a challenge at match committee," Mitchell said.
"I think we've got some (tall) options and then the other one is going a bit smaller at different times, which we've tried at different stages with some success.
"But it's one of those challenges that we'll have to handle week by week now.
"We were always Mitch would come back but now it's going to be a bit further away."
Mitchell is hopeful Dear (ribs) and captain James Sicily (shoulder) will be cleared to take on Fremantle, while small forward Jack Ginnivan (fractured fibula) is less likely to play.
Ginnivan missed the loss to Geelong and sat out Thursday's training session, but could return against his former club Collingwood in round 19.
"When they say 'fracture' you're horrified, but it's a non-weight bearing bone," Mitchell said.
"It's a change-of-direction issue. He can run in a straight line and feel OK, but change in direction has caused him some trouble."
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