Williams also started for Wales on the wing against France with George North moving into the centres but Halfpenny dislocated his shoulder against England, handing the Scarlet his chance.
Halfpenny will miss Wales' summer tour to South Africa following surgery after dislocating his shoulder, ensuring Williams is likely to feature in his favoured position at full-back.
And Easterby is convinced Williams can mount a long-term challenge to Halfpenny.
"[He] really gave the selectors, longer term, something to think about," said Easterby.
"He's been great for us all season but he hasn't had much opportunity for Wales.
"Unfortunately for Halfpenny he gets injured against England and the door opens for Liam to show exactly what he can do.
"He was outstanding and involved with a lot of the positive stuff that happened in the Scottish match."