Tries from France Under-21 wing Mathieu Bourret, flanker Sam Harding and Chris Wyles sealed the Northampton win, but it was Leicester who progressed to the semi-finals after finishing top of EDF Energy Cup Pool D.
Myler's place-kicking may have suffered, with the stand-off missing three conversions and one penalty, but his decision-making, distribution and running in midfield earned him the man-of-the-match award.
"Stephen Myler hasn't played 10 games of rugby union yet but you wouldn't know it," said Grayson.
"He's from good stock - the Mylers have a fine pedigree in rugby league. There's no doubt he can play this game and if he keeps his head down he can go far.
"I can see Carlos Spencer and Myler playing in the same team together, two guys like that, first and second receiver it gives you options in terms of distribution.
"Stephen came looking for us, we were recommended that we looked at this player and it all happened very quickly but I'm glad it did.
"He came well recommended. I was quoted by somebody as saying he's a rugby union player playing rugby league, he accumulates points.
"We are looking for a long-term replacement for Carlos (Spencer) when he moves on and he sees that as his opportunity. His all-round ability to play the game is, I think, fantastic."