Leeds announced their capture of the Scotland international last week as Douglas became their first permanent signing of the transfer window.
The 28-year-old helped Wolves to Premier League promotion last season but what's more impressive is the fee, with Sky Sports reporting that United paid a bargain £3 million for him.
And Warnock, who spent two years at Elland Road until 2015, is blown away by how his former club pulled off such a superb bit of business.
He told The Yorkshire Evening Post: "I think he’s brilliant. I can’t believe they have got him if I’m being honest.
"Hopefully it will be one of the buys of the season at that price. He was up there with the most chances created and assists last season for Wolves and for Wolves to pass him over for £3m is just incredible.
Douglas has since been joined at Leeds by Patrick Bamford and Jack Harrison and finally their transfer window is beginning to take shape.
Director of Football Victor Orta had only brought in two players - Lewis Baker and Jamal Blackman, both on loan - this time last week but huge progress has been made in such a short period of time.
Marcelo Bielsa's side kick off their Championship campaign by hosting Stoke City at Elland Road tomorrow afternoon and Douglas will be expected to start at left-back.