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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Lions get ready for their matches with this green training jersey. It's what the players wear. Get in the Gear!
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.