Being part of a shit defence dosent make you a shit defender. Its an experienced squad player on a free, its a good signing. I doubt very much he's gonna be first choice
Because people love statistics:
In the 3 years he was with Wigan, Alcaraz played 5892 minutes. Wigan scored 69 and let in 93, or 1.05 GF to 1.42 GA per game.
He wasn't on the field for 4368 minutes, almost entirely through injury or suspension. (There was one game where he was sent off for a second yellow, but the goal scored without him occurred after Rodallega also got sent off, so I'm counting that goal here.) In that time, Wigan scored 60 and let in 103, or 1.24 GF to 2.12 GA.
That certainly makes it seem like Alcaraz was really the key to keeping them afloat, as over a full season, that breaks down as 40:54 with Alcaraz, and 47:81 without him. That's basically the difference between floating in the 13-16 position range, and certain relegation.