McLean, 33, and his bandmates have been prepping to hit the road with New Kids on the Block.
"He wanted to go in before they opened their new tour," the source explains.
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Reps for the singer -- he spent time in rehab in 2001 and 2002 for alcohol and cocaine abuse -- had no comment.
McLean Tweeted over the weekend: "I love u all and wish me luck! I start my sober journey [tomorrow]...See u on the other side!"