"He's the most talented player I've ever seen," Martinez told TalkSPORT.
"He never gives the ball away. I catch a cross and I see Grealish running, I know it's a shot on target or a corner – for, not against!
"He will drive past two or three players, I just see Messi with a right foot! When he's on the ball, you can't get it off him.
"I think he's the most talented English player and I'm surprised that he's not played so many games for England.
"Messi is Messi, the best ever. But, in terms of training, when Jack is on the ball people don't want to tackle him, they respect him on the ball.
"He can finish, he can chop, he can do everything. Messi is the same. When I am in the national team and I train with Messi no one wants to touch him.
"He chops people, he uses people like cones and I see that with Jack here so that's why I compare him a little bit."