Duke, who played M'Baku in the MCU film, called Boseman someone he looked up to both "on and off screen," whose "calm confidence was inspiring and exemplary."
The actor's post, showing the two at GQ's 2018 All-Stars Celebration, comes days following Boseman's death on Friday, August 28. Boseman died after a private, 4-year battle with colon cancer. He was 43.
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"How do I start to honor a man who I saw as a giant in many ways; with whom I thought I had so much more time," began Duke. "I am absolutely devastated by the loss of my friend and hero, Chadwick Boseman."
"I feel sick ... I'm in pain and I can't believe he's gone. Chadwick was a lightning rod for me ... he gave me direction," he wrote. "When I saw '42,' I said, I could be like THIS guy. I knew I belonged because I could see myself in you ... that's what heroes do... they seem familiar because they make it possible for us to see our potential best selves demonstrated through them."
"Through seeing Chad’s work, I was able to say, 'I could be just like you one day' and when I saw you in person for the first time ... at my audition for Black Panther, you acknowledged me and my dream by saying, 'he's ready'! Man, your words that day made me feel like my dream was finally real!"
Duke said Boseman showed the cast "what it was to be a leading man," one who "welcomed and created a space for all of us to feel safe, open and bold." He also thanked his late costar for working with him one-on-one on their scenes together, "so that we could get it right, well before the day of filming."
"Thanks for being someone I could look up to on and off screen ... your calm confidence was inspiring and exemplary," he wrote. "Thanks for sharing with me ... you go ahead ... you did your job and did it well! You will NEVER be forgotten. Your heroism is now legend. We'll carry the load and honor your legacy, the rest of the way! Bless King!"
He closed out his post with the "Black Panther" rallying cry, "Wakanda Forever."
Duke was one of the Marvel stars who also paid tribute to Boseman on Sunday night, during a special presentation of "Black Panther."
Following messages recorded at home from Don Cheadle, Scarlett Johansson, Mark Buffalo, Jeremy Renner, Sebastian Stan, Clark Gregg, Tom Hiddleston, Benedict Cumberbatch, Angelina Jolie, Elizabeth Olsen, Kevin Feige, Oprah Winfrey and Simu Liu, Josh Brolin, Whoopi Goldberg, Paul Rudd and Chris Evans, Duke said Boseman was "the last person I thought would leave us."
"You were the last person I thought could leave us so soon," he added. "And I just, my friend, not just my hero, you were my superhero. You were my Black Panther."
See more tributes from the "Black Panther" cast below.