The Dutch midfielder pitched up at Ibrox after a season at Barcelona, staying for four seasons and winning the 2002/03 domestic treble as well as the 2001/02 Scottish Cup.
Answering fans' questions in an interview with FourFourTwo magazine, de Boer was asked if he had really picked the Light Blues over the Red Devils.
"Yeah, that's true," he recalled. "I remember being in the Barcelona kit room, speaking to Alex Ferguson on the phone. He told me he was keen to sign me and explained how he'd use me.
"He said he'd rotate his midfield, and that I'd be competing with Nicky Butt, Ryan Giggs and Paul Scholes, but I'd get plenty of minutes.
"I told him I really liked the idea, but around the same time, Dick Advocaat approached me on behalf of Rangers.
"I asked Louis van Gaal what he thought. He said that at Rangers, I'd always play, which would be better for a me as a 30-year-old, and for my role in tne Dutch team as well."
The former Ajax midfielder also admitted he was concerned he would fail the United medical, adding: "I also chose Rangers because of my knee.
"I wasn't injured, but I thought I might not get through the United medical. At Rangers, I was able to sign for four years, which gave me some certainty.
"Of course, there are times where I think, 'How would I have done [at United]' but it's not like I'm regretting my choice because I had a wonderful time at Rangers."