In 2008, he hit .373 with 15 home runs, 51 RBIs, and eight stolen bases.
In 2008, Alonso was drafted for the second time with the seventh overall pick by the Cincinnati Reds. He signed a major-league contract with the Reds worth $4.55 million through 2012.
Alonso made his Minor League debut on August 26, 2008 for the Class A Sarasota Reds against the Clearwater Threshers. He went 1-for-3 with a double and a walk. In just six games for the A-Advanced Reds that year, he went 6-for-19 with two RBIs.
After the 2008 season, he joined the Waikiki BeachBoys of the Hawaii Winter Baseball league and hit .308 with four home runs and 21 RBIs in 29 games. He had 32 hits in 104 at-bats.