Lewis was stretchered off during Gills' friendly with Bishop's Stortford
Gillingham will be without midfielders Stuart Lewis and Adam Miller when the League One season starts on 8 August.
They both suffered ankle ligament injuries in the Gills' 3-1 friendly win at Bishop's Stortford.
"I'm frustrated and I feel for them both because they've put so much hard work in during pre-season," manager Mark Stimson told BBC Radio Kent.
"Adam's come back from a double hernia and Stuart returned to training early to try and keep his first team place."
Meanwhile another Gillingham midfielder - Mark Bentley - continued his recovery from a groin injury by playing the first half of the team's friendly with Aveley.
The Gills lost the game 1-0, but Stimson declared himself satisfied with the performance, particularly the return of Bentley.
"It was really important that he played those 45 minutes.
I thought he played really well and got in the box like we know he can. He was just unfortunate not to score," the Gills boss told the club's website.
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