Mertesacker, who will take on the role of Arsenal academy boss at the end of the season, was full of compliments for the homegrown prospects after the match.
"You can see that it's going to be difficult against League One teams," the Evening Standard quotes the German veteran as saying. "They are tough, they come at you, every single time you touch the ball someone is behind you. It was a good test.
"To get comfortable playing in front of 60,000 is what we have the development centre for, the academy. That's what they want, so they need to rise to those occasions and play to the level that's required. It's going to be hard, it's going to be tough but those games are vital for those young players."
Arsenal will play host to Norwich City in the last 16 of the EFL Cup.