Musso, 20, took to his page on Twitter.com last week (ends15Jan12) to upload a picture of a sparkler, and told his followers, "Just got my girl the fattest diamond emerald cut fully covered wanna see it?? She loves it. Whatcha think?"
The post prompted gossips to speculate that Musso and Mantegna were set to wed, but the actress has taken to her own account on the site to brush off the rumours.
She writes, "Not engaged! mitchel musso just likes to spoil me silly."
As is tradition, a celebrity's daughter will act as Miss Golden Globe - and the 20-year-old Secret Life of the American Teenager star has been selected for January's (11) event.
She follows in the footsteps of Melanie Griffiths and Jack Nicholson's kids.
Mantegna says, "This has been an on-going tradition for years and I can't believe I'm a part of it so early on in my career. I'm just super excited. It's been very top-secret information for a while, so I think we're all very excited just finally being able to tell everybody."
Grace Gummer, Streep's 23-year-old child with husband Don Gummer, made her stage debut in New York last year (Nov08) in dark comedy The Sexual Neuroses of Our Parents, and now she's making the transition to the small screen to play a 17 year old in Gigantic on the kids cable network Nickelodeon.
Gummer will play the daughter of a famous celebrity couple in the series, which is due to air next year (10). The show will also feature Joe Mantegna's daughter Gia, 19.
Grace isn't the first Streep child to test her acting skills - her older sister Mamie, 26, appeared in the movie Taking Woodstock and epic TV mini-series John Adams.