The 18-year-old was a late inclusion in Leeds' first XI at Swansea in midweek after Liam Cooper suffered a pre-match injury.
It was the right-back's full debut for the Whites after graduating from the club's Academy program, and he impressed in United's 2-2 draw.
Not only that, but Shackleton made a brilliant run and grabbed an assist in setting up Kemar Roofe's first-half equaliser.
The future looks very promising for the youngster, and Douglas, a £3 million signing from Wolves this summer, acknowledged as much in an interview with LUTV.
He said: "He was brilliant. You look at the goal, his directness. He created something from nothing and we're going to need that, some individual brilliance now and again.
"He's a good player, a lot of potential. He's very mature for his age too and the player he showed it, it [the occasion] didn't faze him."
Leeds, who sit top of the Championship, visit Norwich City tomorrow and it'll be interesting to see if Marcelo Bielsa's side can maintain their stunning start to the season.
It remains to be seen whether Shackleton starts again but, after his performance in South Wales, few would be surprised to see him retained in the lineup, though it very much depends on whether Bielsa's available centre-back options after Luke Ayling played in central defence during the week.