Lacy was one of the top running back prospects in the 2013 NFL Draft, with a reputation for agility, strength, and ability to run through tackles.
Lacy was selected in the second round (61st overall) by the Green Bay Packers in the 2013 NFL Draft. He was the fourth running back to be selected in the 2013 NFL Draft. In addition, he was the fourth of nine Alabama players to be selected that year. On May 30, 2013, he signed a contract with the Packers.
Lacy made his NFL debut against the San Francisco 49ers and had 41 rushing yards and a rushing touchdown. He suffered a concussion against Washington Redskins on September 15, 2013. He returned against the Detroit Lions on October 6, 2013. He later rushed for 100+ yards in four games. He had a career-high in rushing in a single game against the Chicago Bears, running for 150 yards on 22 carries with a 6.8 average. Lacy passed the 1,000-yd season mark on December 15 against the Dallas Cowboys when he ran for 141 yards and a touchdown on 21 carries. Lacy finished the season with an impressive 1,178 yards and 11 touchdowns with 257 receiving yards. He set the Packer rookie records for most rushing yards and touchdowns in a season. He was named AP-Second Team All-Pro and was named to the NFC Pro Bowl. The Packers made the playoffs with an 8–7–1 record. In the Wild Card Round against the San Francisco 49ers, he had 81 rushing yards in the 23–20 loss.