During the win against Real Mallorca, Tommy entered the pitch at the Camp Nou wearing the number 43 on his back in the 84th minute, replacing Fermín López.
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Belletti said: "It's a source of pride for me and the entire staff.
"We're always keeping a close eye on everyone... making sure they're progressing every week, every month, for the first team..."
Barca coach Hansi Flick also said after the 3-0 win: "It's good for the club to draw on La Masia. It's our way of doing things.
"We're happy with Tommy Marqués ' debut. He's very confident on the ball and knows what he's doing. I like what I see from him in training, and he can play in several positions."