Mascherano mostly plays centre half. He's been pretty good so far, certainly better than last season (although he's not a starter anymore so gets a lot of rest time). He's still quick and very intelligent. Barca defend similar to us, meaning goalkeeper and defenders have lots to do (ter Stegen has been brilliant by the way). Barca have a great defensive record, they've conceded 5 goals in 16 games.
Overall I think he's been fairly solid, Barca have yet to lose since the league began and Mascherano has played his part in that.