McGrath sustained the injury against Melbourne in round 14 and will have surgery on Wednesday.
Lions general manager of football operations Dean Warren said McGrath’s absence would be felt on the field.
"After performing multiple scans and consulting the experts in this area, we have decided to perform surgery on the injured tendon, to give him the best chance of a full recovery and to be ready for the commencement of the pre-season," Warren said.
"While Ash’s experience and class will be missed on the field, we’ve got a very promising young brigade who we’re very confident will step up and relish the opportunity to play more senior football."
McGrath, 29, played predominantly in defence in 2012 before coach Michael Voss swung him forward against the Western Bulldogs in round 13.
The 188-game veteran kicked five goals against the Bulldogs and had three against Melbourne before being struck down by the injury.
He joins Brent Staker (knee) as a sidelined Lion for the remainder of the season.
Brownlow medalist Simon Black will be monitored during the week to assess his availability for Saturday night's match against St Kilda.
Black was a late withdrawal against the Sydney Swans after injuring his knee in the warm up.
Michael Whiting covers Brisbane Lions news for AFL.com.au. Follow him on Twitter: @AFL_mikewhiting
The views in this story are those of the author and not necessarily those of the clubs or the AFL
