On April 9, 2008, Alexander declared himself eligible for the 2008 NBA draft, and later signed with an agent, forfeiting his remaining college eligibility. Alexander was described as the best athlete on paper at the draft overall, by having the second-most number of 185-pound bench reps (24), the second-highest max touch (12' ½") and the second-fastest three-quarter sprint time (2.99 seconds). He was drafted 8th overall in the NBA draft by the Milwaukee Bucks in June, and was signed by the team on July 8. In the 2008 NBA Summer League, Alexander averaged 9.2 points, 3.6 rebounds and 1.2 blocks per game while starting all five games. In a December 7, 2008 Bucks loss to the L.A. Lakers, Alexander led the team in scoring with 15 points. Following the All-Star break, Alexander saw a streak of four consecutive games where he did not play. However, he rebounded in a loss to the Miami Heat by totaling 13 points, 5 rebounds and 5 assists in 20 minutes of playing time. He averaged 5.8 points for the month of January, the most of his rookie season in a month. In a March 30, 2009 win over the New Jersey Nets, Alexander scored a career-high 16 points. Alexander finished the season averaging 4.7 points in his 12.1 minutes per game.