Just as Backstreet Boys fever is back in full force as the guys gear up to hit the road with New Kids on the Block this summer comes news that BSB's A.J. McLean has returned to rehab.

"For personal reasons, I've checked myself into a rehab clinic," McLean said in a statement to MTV News. "With 2011 being a busy year for Backstreet, I want to be healthy and at my best. Thank you for all of your support and respecting my family and friends' privacy during this time. I'll see you really soon."

TMZ reports that sources close to the pop star say he has been "out of control" as of late due to issues with substance abuse. Reportedly, he was under the influence during much of theBackstreet Boys cruise last month. He allegedly entered rehab, his third time in a decade, because his issues were affecting the planning stages of the NKOTBSB tour, set to begin this June.

A rep for the singer had not responded to MTV News' request for comment at press time.

However the singer did leave some messages for his fans on Twitter that alluded to a possible upcoming stint in rehab. "Thank u all for my bday wishes and tweets:)! Its gonna be a great year! I love u all and wish me luck! I start my sober journey tomm! Ttfn!" he wrote January 9. One day later, he added, "Hey all wish me luck! This is gonna be an amazing year for me and all around me! I love u all and appreciate all the love and support"

McLean previously entered rehab in July 2001, just as the Backstreet Boys planned to kick off their Black & Blue world tour. He also went to rehab the following year in 2002.

"[A.J.] came to us yesterday and said, 'Guys, I need help,' " group member Brian Littrell told MTV News in 2001. "I looked at him in the eyes and said, 'I'm proud of you.' That's the first time he said it to any of us: 'Guys, I have a problem, and I wanna better myself and better the group and better our situation.' "