HOUSTON, May 18 Texas Rangers third baseman Hank Blalock will undergo right shoulder surgery Monday and is expected to be out 10-12 weeks.The team described his injury as Thoracic Outlet Syndrome, which causes pain in his right arm.Dr. Keith Meister in Dallas made the diagnosis after an examination Friday."It's a huge loss, especially when Hank was returning to the form he had in the days when he was hitting the ball out of the ballpark and hitting for a high average," said Manager Ron Washington before the start of Friday night ' s game at Houston. "We are going to miss him a lot." Blalock was batting .285 with five home runs, 16 RBI and 144 at-bats in 39 games.He is being replaced by third baseman Travis Metcalf from Class AA Frisco.The much-improved Metcalf was batting .294 with six home runs and 25 RBI in 136 at-bats. Last season, he batted .221 with eight home runs and 37 RBI.Copyright 2007 by UPI