Yi was not expected to enter the NBA Draft until 2009 because the Chinese Basketball Association ruled that players would not be allowed to leave until they turned 22. In early 2006, however, Yi announced that he would enter the 2006 NBA Draft, although he eventually decided to withdraw, saying he was “not good enough to compete in the NBA and needed more experience”. A year later, on November 1, 2006, the Tigers announced that Yi would enter the 2007 NBA Draft.

Yi chose NBA agent Dan Fegan to represent him in the NBA Draft and flew to Los Angeles to participate in pre-NBA Draft camps. Before the draft, Yi was predicted to be picked anywhere from third to twelfth. On June 28, 2007, Yi was selected by the Milwaukee Bucks with the sixth overall pick in the 2007 NBA Draft, despite Fegan warning the Bucks not to pick Yi and not allowing Milwaukee to be one of the teams invited to Yi’s pre-draft private workouts in Los Angeles. He did not want Milwaukee to select Yi because they did not have a large Asian-American community. However, general manager Larry Harris said they had only drafted the best player available to them.

After the draft, the Bucks attempted to convince Yi to sign with the team. On July 2, the owner of the Bucks franchise, Senator Herb Kohl, wrote a letter to Yi and his representatives, hoping to persuade Yi to sign with the Bucks. Three days later, Harris and head coach Larry Krystkowiak met Yi in Las Vegas, desiring to influence Yi to play for Milwaukee; however, Yi’s representatives requested that the team trade Yi to another city with a large Chinese presence. Chinese officials also required that any team Yi played with would have to give him sufficient playing time for him to improve before the 2008 Summer Olympics.

Kohl made a special trip to Hong Kong to appeal to Yi personally.He also assured Chinese officials that Yi would have sufficient playing time, and on August 29, 2007, the Milwaukee Bucks and Yi agreed to a standard, multi-year rookie contract.

After being named to the Bucks’ starting lineup by Krystkowiak in place of Charlie Villanueva to begin the 2007-08 NBA season,Yi had two points and two rebounds in a loss to Charlotte in his NBA debut. He played his first home game in Milwaukee in the next game and scored 16 points while taking eight rebounds in a 78–72 win against the Chicago Bulls. The game was also Yi’s first game to be televised nationally in China, where it was watched by 100 million viewers. Yao Ming praised Yi’s play in his first games, saying, “If you compare us in our third NBA games, you will see that Yi’s statistics are far better than mine”.

On November 9, Yi played against Yao for the first time when the Houston Rockets hosted Milwaukee in the Toyota Center. Yi had 9 rebounds 19 points, including two three-pointers, but the Rockets topped the Bucks 104–88, with Yao scoring 28 points and grabbing 10 rebounds. The game was also watched by over 200 million people in China, making it one of the most-watched games in NBA history. After the game, Yao called Yi’s talent “unbelievable”, and Tracy McGrady said that Yi had “A tremendous upside in this league”.Del Harris, the coach of the China’s 2004 Olympics basketball team, also described Yi as “the most athletic 7-footer in the NBA”.

Yi was named the Rookie of the Month for December after averaging 12.1 points and 6.6 rebound per game in that month, and scoring a career-high 29 points on 14-of-17 shooting on December 22, 2007.On January 30, he was selected for the rookie team in the Rookie Challenge at the 2008 NBA All-Star Game. On February 2, 2008, Yi faced Yao for the second time when the Bucks played the Rockets in Milwaukee, which Krystkowiak dubbed it the “Chinese Super Bowl”. However, both players struggled during the Rockets’ 91–83 victory. Yao had 12 points, and Yi injured his shoulder during the game, finishing the game with 6 points and scoring on one of his ten field goal attempts.