The Hull City loanee has had his best run of form in a Lions shirt in recent weeks, contributing four goals in his past five outings, including the cross that set up the goal at Sunderland on Saturday.
He also created one of Millwall’s most memorable moments of the season earlier this month, scoring an incredible winning goal against Leicester City.
His loan agreement expires at the end of the season, but Harris said he has loved watching the 23-year-old thrive.
The Lions manager told NewsAtDen: “It takes time for me to learn about players and for them to grasp what I want.
“I’ve had an immediate impact on some athletes, while others have required more time. Maybe Longy just needed some time.
“Ultimately, Longy plays his greatest football when he has room to attack. He has pace, energy, and produces a high-quality final product. He has the ability to cross the ball on the run, which is becoming increasingly rare in today’s game. And Millwall supporters appreciate a winger who crosses the ball on the run.
“They loved Jed Wallace, Jimmy Carter and Christophe Kinet.
“Ryan should be really proud of himself. I’ve just tried to help him develop as a Millwall player to help us and he has done.”
