The Man City man got the chance to see Greenwood up close and personal with the Three Lions during this international break.
Greenwood, who came on for Harry Kane during his team's narrow win over Iceland, has impressed in training.
"It's brilliant for English football," Walker said of Greenwood, who is only 18.
"Obviously, Phil [Foden] was involved in the World Cup win for the Under-17s. They're proving it at club-level, with big clubs, so I've said it before the world is their oyster, especially for Phil.
"For Mason, I've not seen much of him, but he seems to be a very good player. He's sharp, and he can finish – I'll tell you that – I've seen him in training."