The two-time All Star, who won the World Series with Boston in 2004 and 2007, is scheduled to have surgery on Friday, the team said on its website (http://boston.redsox.mlb.com).
Youkilis, 31, is the Red Sox's second-leading hitter this season with a .307 average. His absence deals a blow to a Red Sox team sitting six games back of the New York Yankees in the American League East division.
"We lose a big part of our heart and soul and one of the best hitters and best players in the league," Boston General Manager Theo Epstein said. "That's not the type of guy you can replace."
Youkilis was injured while taking a swing in Monday's game against Cleveland in Boston. He had hoped it was a jammed thumb but follow-up tests revealed the adductor muscle tear.
(Writing by Gene Cherry in Raleigh, North Carolina; Editing by Frank Pingue)