The Detroit Lions waived Greenwood on Monday, four days after he was injured in their 24-6 exhibition loss to Cleveland.
According to reports out of Detroit, Greenwood suffered a neck-stinger injury but a source told The Canadian Press that Greenwood actually has a shoulder ailment that will require season-ending surgery.
As a result, it's likely Greenwood will clear waivers, after which the Lions are expected to put the 28-year-old Canadian on their injured reserve list.
Greenwood was entering his fourth NFL season but first with Detroit.
Greenwood signed with Kansas City as a free agent in 2010 and spent three seasons with the Chiefs, mainly on special teams, before being released in May.
Greenwood signed with Detroit less than a week later.
Greenwood has amassed 35 career NFL tackles and will qualify for the NFL's pension play with three more regular-season appearances.
Greenwood's CFL rights belong to the Grey Cup-champion Toronto Argonauts, who selected him third overall in the 2010 draft out of Concordia.