Lopez was drafted by the New Jersey Nets 10th overall. His twin brother, Robin, was drafted 15th overall by the Phoenix Suns. Lopez played in his first NBA game on October 29, 2008 in a 95-85 Nets win. In his first game Lopez came off the bench and had 8 points 8 rebounds and 2 blocks in 25 minutes. Lopez began starting for the Nets after an injury took Josh Boone out of the line-up, and afterwards he started putting up solid numbers for the team. Lopez was selected to compete in the 2009 NBA All-Star Weekend Rookie Challenge. For his rookie season Lopez averaged 13.0 points, 8.1 rebounds and 1.9 blocks per game, playing in all 82 games of the regular season. He ranked 4th in the NBA in total blocks with 154 and 9th in blocks per game with 1.9, which was first among rookies. He finished third in the NBA Rookie of the Year voting and was named to the All-Rookie first team. He was selected as NBA Rookie of the Month twice in the months of January and February. The Nets finished at 34-48, failing to qualify for the playoffs.