Lopez, 30, will sign a one-year deal for the biannual exception of $3.4 million, the sources said.
Milwaukee is expected to pencil in the 10-year veteran as its starting center.
The 7-footer played last season for the Los Angeles Lakers, where he averaged 13.0 points and 4.0 rebounds in 74 games. Over the past two years, he has expanded his game to become a 3-point threat.
This will be the third NBA franchise the one-time All-Star has played for. He has a career average of 17.9 points and 6.8 boards.
The Bucks still have to deal with the matter of forward Jabari Parker, who is a restricted free agent.