"Avengers 4" will feature a blast from Captain America's past.
Grillo himself confirmed on the UFC Unfiltered podcast that he'll appear in "Avengers 4."
"[Crossbones] makes an appearance in the next 'Avengers' movie, but it’s a flashback," he teased.
"And I’m allowed to say whatever I want because I’m never doing another Marvel movie … because I’m 117 years old."
Crossbones was an antagonist in "Captain America: Winter Soldier" and "Captain America: Civil War." Rumlow initially posed as a S.H.I.E.L.D. operative, but was actually working for the nefarious HYDRA. After his body was burned in "Winter Soldier," he wore mask.
At the beginning of "Civil War," he tried to kill himself and Captain America in an explosion, which Scarlet Witch was able to contain but ended up taking out Crossbones.
Grillo is certainly not afraid of revealing spoilers or rumors about the Marvel Cinematic Universe. Earlier this week, he told Larry King that the next "Captain America could be African American. It could be a woman."