Team owner and president Jamie Dawick made the announcement Thursday in a statement.
"On a recent family vacation, Terry became ill and will not be in a position from a health standpoint to continue his duties as general manager of the Toronto Rock for an undetermined amount of time," Dawick said. "While Terry works his way towards making a full recovery, I will be assuming his day-to-day responsibilities as GM.
"I will continue to work closely with head coach John Lovell and his coaching staff and continue to be diligent in putting together the most competitive team possible."
Sanderson, a 62-year-old resident of Orangeville, Ont., guided Toronto to Champion's Cup titles in 2005 and 2011. He has over 40 years experience as a player, coach and manager.
"The Sanderson family is very appreciative of the well-wishes they have received thus far and at this time we would like the Toronto Rock and NLL communities to continue to respect the privacy of the family," Dawick said. "The team will not be making any further comment on Terry's health.
"As the lacrosse family is a very close one, we hope everyone will keep Terry and the Sanderson family in their thoughts and prayers."
The Rock will kick off their 2015 season on Jan. 3 at Rochester.