Robert Downey Jr. doesn't know what he is going to do when Chris Evans isn't Captain America anymore.

The 52-year-old actor kick-started the Marvel Cinematic Universe with the 2008 movie 'Iron Man' and has since reprised his role in six films and is set to return in 'Avengers: Infinity War' and 'Avengers 4'.

However, Evans, 36, is reportedly hanging up the role after the end of 'Avengers 4' due to his contract coming to an end, and Downey - whose contract is also ending - hasn't confirmed whether he will no longer star as Iron Man, and has nothing but praise for his co-star.

He told Comicbook.com: ''I feel like my instinct would be to bust his jaw.

''All I know is that there's something about being on camera with Evans. I hate to say it, but I don't know what I'm gonna do when he's not Cap anymore.

''I look at him and think this is the truest, most down moral psychology I've ever seen and then they go, 'Cut!' and Chris is like, 'Hey buddy, I gotta get out of here'.

''I'm like, 'You wanna go to dinner?' 'Nope'. 'Okay! Love you! Love you, Cap! I mean, Chris!'''

Evans also recently said how he doesn't think anyone else but Downey could play Tony Stark aka Iron Man.

He said: ''It's low-hanging fruit to say [Robert] Downey. He's such a wonderful guy and is so talented, so experience, and so supportive.

''He's always been in my corner and I've always felt supported by him. He really brings everybody in to the group and makes sure everyone feels welcome, especially on this last one.

''He really just opens his arms and it sounds so cheesy, but he makes it a family and none of this would happen without him.

''No matter what it feels like for me, I try to imagine what it feels like for him, because he really did start this. He really is so irreplaceable.

''Nobody can ever be Iron Man. It's not a role like Superman or Batman that can kind of find different incarnations, no one can touch it.''

Both actors will be reprising their roles in 'Avengers: Infinity War' which is slated to be released this year.