Jay-Z is concerned that fame has gone to Rihanna’s head — so he’s hired a bodyguard to keep an eye on his protege, The National Enquirer has revealed.

The hip-hop heavyweight plucked the Barbados-born beauty from the island in 2004 and “invested his heart and soul into transforming Rihanna into a superstar,” but her behavior in the months since her brutal brawl with ex Chris Brown has concerned Hov so much that he recently assigned one of his bodyguards to look after the 21-year-old.

“Jay-Z’s take is that Rihanna’s been on a downward slide every since the Chris Brown beating. He thinks she’s losing her edge,” a tabloid snitch tells The Enquirer.

After last month’s MTV Video Music Awards, Rihanna danced the night away at NYC hotspot 1OAK and Jay-Z’s 40/40 Club, and she’s often spotted tossing back cocktails at M2 Ultralounge in the Big Apple.

“He wants to keep an eye on Rihanna to make sure she doesn’t wind up in trouble,” says the source. “He’s umhappy that she appears to prefer partying to making music.”