He told nufcTV: "Sometimes people say that his delivery lets him down but against PSV it was top class. He's played up front but he could play anywhere, if you asked him to play full-back he could.
"He's worked extremely well and has never stopped running."
And, with Milner not due to turn 23 until next year, the Newcastle boss firmly believes that we're yet to see the very best of the former Leeds United starlet.
"He's a gem of a player," added Keegan. "Everyone at the club loves him. He works hard in training and wants to improve himself.
"And I really think we are yet to see the very best of James Milner and that could be very exciting."