"It is good to see how much he has improved in the last 18 months and going to Liverpool will improve him even more. Their coaching and the whole club can only be of benefit to him," said the Welsh defender.
"It will take his game on even more and that can only help Wales with him on form and firing up front.
"He has been on fire at the start of the season and that can only be good for Wales. He is a fit lad, he can run all day and he has that skill to be a good finisher.
"He could easily play up front on his own for us the way he plays with that energy. Thankfully he's on my side at international level.
"I had only played against Craig once before, and that was the first game of last season when he was nowhere near fit when West Ham started their top-flight campaign against Blackburn.
"But it was a good battle on Saturday, he's a great player and I love facing the best because it only improves my game.
"It was a good competition between us. I know all about his pace and what he can do to a team, so it will be good to have him at the sharp end for us rather than coming at me.
"I had to be wary, I didn't want him to get in behind us. He managed that in the second half because we were chasing the game and taking more chances.
"He played really well. We decided to play offside against him and push up and he was getting caught a lot. He got a bit niggled and I was happy with that because he was spending too much time arguing with the linesman."