Stallone, 70, showed up briefly in the footage shown at Comic-Con, appearing in just a single one-second shot, reported Digital Spy..
"It's a very, very key, important role. It's not a huge role. He's a great and key supporting actor," Gunn said.
Meanwhile, rumours that Kurt Russell is Star-Lord's (Chris Pratt) father were confirmed at Comic-Con, with the actor playing Ego the Living Planet in the 2017 sequel.
Zoe Saldana, Dave Bautista, Vin Diesel, Bradley Cooper, Michael Rooker, Karen Gillan, Sean Gunn, Glenn Close, Pom Klementieff, Elizabeth Debicki and Chris Sullivan will also star in the movie.