Cook was selected with the 24th overall pick in the 2003 NBA draft by the Los Angeles Lakers. He played sparingly in his rookie campaign as a backup to superstar center Shaquille O'Neal, and was mostly an interior player, notching only five three-point attempts out of 141 total field goal attempts. As his professional career evolved, however, Cook once again became a player who could play beyond the perimeter, after O'Neal moved on to the Miami Heat and Rudy Tomjanovich took over for Phil Jackson as Lakers' head coach. He was mainly used as a three-point specialist; 199 of 422 (or 47.2%) of Cook's field goal attempts were from behind the three-point arc. When Phil Jackson returned to coach the Lakers in 2005–06, Cook started to take less three-pointers again, which resulted in Cook improving his overall field-goal percentage from .417 in 2004–05 to .520 in 2005–06. It also resulted in an improvement in his three-point field-goal percentage, from .392 in 2004–05 to .441 in 2005–06. His improved play kept Cook on the court more as his minutes played per game rose from 15.1 in 2004–05 to 19.4 in 2005–06.