The team said late Tuesday night that Parker will have surgery after tests revealed the extent of the injury.
The 19-year-old Parker was hurt when he took an awkward step before colliding with the Suns' P.J. Tucker. It's a huge blow for a rebuilding franchise off to a good start under new coach Jason Kidd.
The Bucks, who won 15 games all of last season, are 13-12.
Parker, the second-overall pick in this year's draft, averaged 12.3 points and 5.5 rebounds per game this year.
Parker was a one-and-done star at Duke before getting drafted by Milwaukee in June. The polished forward was a cornerstone in a rebuilding plan for a franchise that had finished above .500 just once since 2003-04.
New York investment firm executives Marc Lasry and Wesley Edens bought the team in April from former U.S. Sen. Herb Kohl. They hired Kidd soon after drafting Parker.
"Tough break but (Parker) will come back stronger and hungrier. Please join me in wishing him a speedy recovery," Lasry's son, Bucks executive Alexander Lasry, wrote on Twitter.
Milwaukee next plays Wednesday at Portland.