The Spanish side are yet to concede in the league this term, with summer signing Mathieu impressing during the early stages of his Barca career.
"The important thing is that in the three games we've played we didn't concede a goal," he told AS.
"The reception from the team has been amazing and made me feel very comfortable. It's complicated to prepare for a game when the players have been with the national teams.
"We knew we had to be patient to overcome Athletic, a very strong team. We lacked a bit of pace. If we continue like this we can be up there. The defenders talk a lot to avoid mistakes and our attack is tremendous."
Mathieu moved to the Camp Nou from Valencia this summer.