Previously known as Bill Walker,[1] the 6-foot-6-inch-tall (1.98 m) player was selected out of Kansas State University by the Washington Wizards with the 47th overall pick in the second round of the 2008 NBA draft and traded shortly thereafter to the Boston Celtics in exchange for cash considerations.

In 2006, North College Hill finished third in the USA Today Super 25 High School rankings,[6] with Walker averaging 21.7 points and 10.1 rebounds per game.

In the fall of 2006, Walker graduated early after it was ruled that he had used all of his high school athletics eligibility the previous year.

After receiving interest from many NCAA teams, including Cincinnati, Connecticut, Illinois, Syracuse, Southern California and Texas, Walker enrolled at Kansas State University as a part-time student in order to be eligible to play on December 16, 2006, the start of the second semester.

However, he ruptured the anterior cruciate ligament (ACL) in his left knee early in a game against Texas A&M on January 6, 2007, which required surgery and abruptly ended his freshman season.

[9] Many considered Walker a lottery pick,[10] but he suffered the third knee injury in his career during a workout[11] at the Golden State Warriors facility on June 15, 2008.

[15][16] In November 2008, Walker was sent to the Celtics' NBA Development League affiliate, the Utah Flash,[17] and was recalled on January 8, 2009.

[20] On February 18, 2010, Walker, along with J. R. Giddens, Eddie House and a future conditional second round draft pick, was traded to the Knicks for Nate Robinson and Marcus Landry.

[32] On June 8, 2016, Walker signed with the NLEX Road Warriors of the Philippine Basketball Association as the team's import for the 2016 PBA Governors' Cup.