Butler, 33, started for the Los Angeles Clippers the last two seasons.
Butler, who is due almost $8 million next season, averaged 10.4 points, 2.9 rebounds and 24.1 minutes in 78 games last season. The Clippers dealt Butler to the Suns in July as part of a three-team trade that also involved the Bucks.
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The Bucks, meanwhile, are committed to trying to make the playoffs for the second straight season. After a flurry of offseason trades and free-agent signings, they appear set at every position except small forward, which is where Butler plays. Carlos Delfino, whom the Bucks signed as a free-agent in July, had been the projected starting small forward. But Delfino is still recovering from surgery for a fractured bone in his right foot.