NFL Network Insider Ian Rapoport and Mike Garafolo reported Friday that Murphy suffered a torn ACL in Thursday's preseason game in Baltimore.
Murphy earned a career-high nine sacks in 2016 as a rotational pass rusher. The 26-year-old didn't start a game last season, but played 45 or more snaps in 12 games, mainly in passing situations.
Murphy was slated to miss the first four games after getting popped for a PED suspension in April.
The injury wipes away the season for Murphy and dings what looked to be an improved front seven for the Redskins. Washington added first-round defensive end Jonathan Allen, second-round linebacker Ryan Anderson, and veteran linebacker Zach Brown to a group led by Ryan Kerrigan and Preston Smith.