At the other end of the spectrum is Faletau, who will make his debut in this year's competition after impressing in the back-row at the World Cup in New Zealand.
But despite heading into the unknown Williams is convinced the Newport Gwent Dragons starlet has what it takes to be a key figure in their trip to the Aviva Stadium.
"He is phenomenal, the most naturally gifted rugby player I have ever played with," he said of the 21-year-old.
"A lot of the time you get great athletes who are not perhaps the most natural rugby players, and vice-versa. But he has got the whole package.
"After he first came into the Wales set-up for the Barbarians game, Toby worked really hard with the conditioning coaches.
"He has just been unbelievable since, particularly when you take into account that the World Cup was his first major tournament. Everything comes so easily to him.
"When you see him playing in a sometimes struggling Dragons side, you can imagine what he would do in one of the top teams. But he is the sort of player you can build a franchise around, he is that good.
"He is very quiet but I think he just loves the game of rugby, he loves training, and he has been in outstanding form since the World Cup."