"So sad watching him," wrote one.
"Nothing but respect for Mr Fox," tweeted another. And another posted: "Lump in the throat time. Tragic to see the decline, brave man though."
Meanwhile, a fan pointed to Fox's appearance as a reminder that perhaps things aren't so bad for many of us: "When you feel all down on yourself think of Michael. If he can go on, so can you. Count your blessing [sic]. Life ain't so bad."
On stage, Fox reminisced about working with Lloyd, sharing that their time together was the best part of making Back to The Future.
He described his co-star as "just genius" and said the pair had "immediate chemistry".
Lloyd was equally appreciative of his time working with Fox, who he called the "King of Exposition".
Fox also addressed his health and spoke about his charity the Michael J. Fox Foundation - the biggest Parkinson's disease foundation in the world from which he has reportedly raised over US$1.5 billion for research into the disease.
He told the enamoured Comic Con audience: "Parkinson's has brought people who are still coming in, but it's a gift, and I wouldn't trade it for anything.
"People like Chris have meant so much to me, and so many of you have too.
"'It's not about what I have, it's about what you've given me - a voice to do that and help people."
In June, Fox revealed the impact Parkinson's disease has had.
"When I did the spinoff from The Good Wife, which is The Good Fight, I couldn't remember the lines. I just had this blank, I couldn't remember the lines," he revealed in an episode of Mike Birbiglia's Working It Out podcast.
He has been forced to avoid projects with a lot of lines: "I can't remember five pages of dialogue. I can't do it," he said. "So I go to the beach."
In 2020 he also revealed how the disease has affected his memory.
"My short-term memory is shot. I always had a real proficiency for lines and memorization. And I had some extreme situations where the last couple of jobs I did were actually really word-heavy parts. I struggled during both of them."
