"Since coming in and building in pre-season I think Craig has come on leaps and bounds," Appleton told the WVA website.
"He's shown that in the Under-21s games. I think those games are a little bit different to league football.
"Obviously going into the Premier League is another step up again.
"International football is a little bit slower tempo and gets really quick around 30 yards from goal. But he did get tested in the two games at various times and although they were two decent wins in the end for England it is still great experience for him.
"He's been training well with the team and has been improving all the time. He's a young lad who is a good athlete and is always wanting to learn and listen."