The 31-year-old started 29 games last year for the club, with his form earning him the captaincy in the latter part of the season.
However Newby has now undergone surgery to repair a knee problem.
Tigers director of rugby Richard Cockerill told the club's official site: "Newbs has had a bit of a problem with the knee and it was decided that he would require an operation.
"We expect to be without him for at least the opening phase of the season while he comes through a thorough rehabilitation programme.
"It's a blow for Newbs and for the club because his experience and his leadership have proved very important for the squad.
"He enjoyed an excellent season in 2010/11 and we were looking for him to play a big part again this year, especially as we will be missing a number of our experienced players and leaders due to the Rugby World Cup."
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