Per Sean Deveney of Sporting News, Butler is "uncertain" about playing with Wiggins because of "his work ethic and his approach on the defensive end of the floor."
Deveney noted T-Wolves head coach Tom Thibodeau has had similar concerns with Wiggins, and things didn't improve with Butler added to the mix.
Wiggins signed a five-year max extension with the Timberwolves in October. Two months prior, team owner Glen Taylor told the Associated Press he wanted to see more out of his 23-year-old star.
"When you're talking about negotiations on his part, I'm already extending to him that I'm willing to meet the max," Taylor said in August. "But there are some things that I need out of him, and that is the commitment to be a better player than you are today."
Last season was a step back for Wiggins, who saw his scoring average drop from a career-high 23.6 in 2016-17 to 17.7 per game.
The Timberwolves made the playoffs last season for the first time since 2003-04, but the Houston Rockets eliminated them in the first round in five games.