The portion of the show that features the mentors has already been filmed, and Gomez, 23, apparently did an incredible job.
"She exceeded everyone's expectations!" added the source.
NBC subsequently confirmed Paisley will definitely serve as a mentor, according to People.
"He's really been good to me, and so the fact that I was able to get him here and advise my team made perfect sense to me because he was actually a mentor for me," Shelton told People.
"If you know anything about country music, you know who Brad Paisley is. He's entertainer of the year. He's male vocalist of the year. He's sold millions of albums, and sold out tours. He's just one of the more respected singers and just people in the music industry and Nashville."
Paisley, 42, appeared on The Voice's inaugural season when he and Shelton, 39, performed their duet "Don't Drink the Water."
"I wouldn't want to do this with anyone else -- he's a blast to be around," Paisley said of Shelton. "We debuted [in country music] roughly around the same time... And we toured together in the struggling years. We have some great memories that I wouldn't trade for anything."
The Voice's ninth season is set to premiere September 21 at 8PM ET/PT on NBC.