The 60-year-old music mogul was forced to pull out of the series after he broke his back while riding an electric bicycle in Malibu earlier this month.
Though Simon is recovering from the subsequent operation, which left him with a steel rod inserted in his back, he will not be fully healed in time to judge on the 2020 series of BGT.
Ahead of his BGT debut, the 31-year-old spoke out on the replacement, saying: "First and foremost, me joining the show this year is bittersweet because I feel like this year has been full of horrible news.
"There are a few people over the course of Diversity's career who have been integral and had a huge impact, and Simon's one of those.
"So, to hear that he had had the accident was horrible. I'm just so glad to hear he's recovering well."
Ashley was one of the original BGT champions, as legendary dance crew Diversity won the third series in 2009, only to skyrocket to fame in the years that followed.
The dad-of-two admitted he was feeling a little nervous filling Simon's shoes, insisting he wouldn't try to compete with the Syco boss' onscreen presence.
He said: "That's the thing, if I was stepping in for any of the other judges then I'd feel that way, I'd feel nervous about it being big shoes to fill.
"But with Simon his shoes are well and truly on, no one can ever replace him or fill his shoes. I just feel like I am doing a job for him because Simon can never be replaced in that way.
"I'm just an addition to the panel, that's the way I see it. It actually makes me less nervous, the fact I'm stepping in for Simon."
Praising Simon's legacy, Ashley added: "There's no one more qualified than Simon when it comes to judging variety acts, and if you're going to sit on that panel you better have some decent qualifications. "
Luckily for the contestants, Ashley isn't likely to channel Simon's "Mr Nasty" reputation, and instead plans on bringing constructive criticism to the table.
He explained: "I'll be tough on everyone! I won't be Mr Mean, just Mr Honest. With a show like BGT there's no mucking about. It's not a celeb-based show, it's not like everyone's there just for entertainment, this is life-changing stuff.
"I'm going to be sitting there questioning what they are doing, why would you use your two minutes to show the world what they can do and then do that? If that's how I feel, then I'll say it!"
Britain's Got Talent will debut on Saturday at 8pm on ITV .