The 21-year-old centre-back, who had been sidelined since November following ankle surgery, started the Premier League 2 game for the young Reds.
Phillips joined England international Alex Oxlade-Chamberlain in making a return to action at Derby's Training Centre - and was pleased with his comeback.
"I'm really happy to have got 45 minutes under my belt," the defender told Liverpoolfc.com.
"It was important for me and I enjoyed myself out there, I'm so happy. I just want to get as fit as possible now and get back to full fitness."
Neil Critchley's side made a return to winning ways courtesy of two goals from Curtis Jones and another by Rafael Camacho.