Fitzgerald told ESPN on Monday that he was initially "really taken aback" by the request of Cindy McCain, who asked the 34-year-old wideout to speak at the service a couple of months ago. John McCain died on Saturday after a battle with brain cancer. He was 81.
"It's truly one of the greatest honors of my life to be able to talk about somebody who served his country so passionately and had been so dedicated to policies of this nation," Fitzgerald said. "So, I was really taken aback by it."
Fitzgerald said he'll have five to six minutes for his tribute.
"Fortunately, I've had some time to get prepared and put some of my thoughts on paper," Fitzgerald said. "I've had to kind of shorten it down. You have a short window. It's tough to be able to get everything you want to say. How do you accurately be able to give somebody their due justice who served the United States for 60-plus years, I mean dedicated his entire life.
"You think about those 60-plus years and everything he's done from his time served as a POW to all the years in the senate, the things he's done her for the great state of Arizona -- and to be able to do that in five or six minutes is extremely difficult. I'm going to do the best I can and try to honor him for the wonderful man that he is."
McCain, who was severely injured after his plane was shot down over North Vietnam in 1967, spent more than five years as a prisoner of war.
He was later elected to the House of Representatives in 1982, the Senate four years later and made an unsuccessful bid against Barack Obama in the 2008 Presidential Election.
Fitzgerald, who penned a tribute to McCain for Sports Illustrated in December, took to social media to honor his "dear friend" on Sunday.
"Rest in peace to an American hero, statesman, servant of the people, and dear friend," he wrote. "Godspeed Senator McCain. My prayers for Cindy and the beautiful McCain family."
Fitzgerald visited McCain in February in the wake of the latter being diagnosed with glioblastoma, an aggressive form of brain cancer.
Cindy McCain shared a picture on her Instagram account of the three of them at the cabin in Cornville (Ariz.), where the elder McCain had been receiving medical treatment.