Ricciardo announced his shock exit midway through the 2018 season, joining Renault for the subsequent two years.
At the time, Red Bull had what was widely-regarded as the strongest driver pairing on the grid in Ricciardo and Max Verstappen.
However, after the pair’s crash at the 2018 Azerbaijan Grand Prix, Ricciardo felt the team were slowly starting to back Verstappen.
Ricciardo was also disgruntled by the fact Verstappen was handed a bumper new deal earlier in the year.
Ultimately, the move to Renault - and then McLaren - didn’t work out as Ricciardo could only pick up a solitary win and a couple of podiums, before returning to the Red Bull stable with AlphaTauri.
Horner confirmed that Ricciardo quickly realised “he made a mistake” by leaving.
“I think he recognised that he made a mistake. He had no good advice around him at the point that he left us,” Horner said on Dax Shepard's Eff Won podcast.
“Having spent a few years outside of the family he suddenly realised that, 'Whoa, actually what I had was really good'. Actually it was during the pandemic, I remember he called me and he said, 'Christian, I hate to say this to you, but you were absolutely right, and I apologise' and so on.'"
Horner believes Ricciardo was “very badly advised” during that part of his career.
“There was an element of [spite] at the time, thinking, 'You know what? Okay. Go and suck on the lemon for a bit',”Horner said. “He was very badly advised in his earlier career. Everybody f***s up at some point.
“We gave Max a contract at the beginning of that year in 2018 to secure his future. Daniel, I remember being upset at the time. He suddenly felt that, 'Hang on, I don't want to be the support act here'.
“I could tell he was starting to think of being a bigger fish in a smaller pond. He got a lot of noise in his ear that this is the Max show. Money (from Renault) was also on the table.”