Keough, who is Elvis Presley's granddaughter, was spotted with a ring on her engagement finger earlier in this month (Mar12), prompting speculation she is planning to wed the British actor after meeting him on the set of the new male stripper movie six months ago.
Reps for the pair have yet to respond to the engagement rumours, but Manganiello, who appears in the film alongside the couple, reveals he saw the news coming while they were working together on set.
He tells E! News, "If that's true, then I'm very happy for them... I love both of them. I think we all (knew)."
Pettyfer has previously dated Dianna Agron and Emma Roberts.