Newby, 31, was one of the club’s most consistent performers last season, settling into the No7 shirt, and he captained Tigers in the second half of the season, including the Aviva Premiership Final at Twickenham in May.
Tigers director of rugby Richard Cockerill said: “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.”
Newby’s total of 29 starting appearances in 2010/11 was topped only by fellow New Zealander Scott Hamilton in the Tigers squad.