Coach Jason Kidd said after the game that Parker will be further examined Tuesday.
Parker's knee appeared to buckle on a drive to the basket before he collided with the Suns' P.J. Tucker, and Parker was unable to get to his feet immediately afterward.
Parker, the Bucks' first-round draft pick this year and the second player chosen overall, had to be carried off the court by teammates to the locker room.
"I hope he's going to be OK," teammate Zaza Pachulia told reporters after the game. "He's a great young player. This team and this organization, the whole city counts on him. He has a lot of years ahead of him in his great career. Injuries are part of the game.
"It was tough to see your teammate going down and not being able to walk himself. We'll see what happens tomorrow. The doctor is going to make a decision, obviously. But we want to him to have a speedy recovery, whatever it is. We really need him."
Parker had eight points and three rebounds in 15 minutes. He's averaging 12.5 points and 5.6 rebounds per game.
The Associated Press contributed to this report.