As Philadelphia Eagles doctors were giving newly acquired running back Jerome Harrison his physical, they discovered a brain tumor that nullified a trade with the Detroit Lions, according to two league sources.

The trade might have actually saved Harrison's life, the sources said. Without the deal being made, Harrison would not have undergone a physical. The tumor is now being treated, according to sources.

Harrison could not be reached for comment. The Lions have put him on the reserve/non-football illness list, replacing him on the roster with running back Eldra Buckley.

The Eagles would have sent running back Ronnie Brown to the Lions in exchange for Harrison and an undisclosed pick in the 2013 NFL draft. Brown arrived in Detroit but never got to practice Wednesday.

Harrison is not expected to play again this season, but his long-term prognosis both for life and his football career appear to be good, sources said. He is visiting with more doctors Thursday.

The voided trade has left the Lions with major issues at running back, after Jahvid Best suffered the third concussion of his career in last week's loss to San Francisco.

Adam Schefter is ESPN's NFL Insider. Information from ESPN.com's Kevin Seifert was used in this report.