The 29-year-old, who moves to the club from Hercules CF, is Di Matteo's sixth new signing of the summer.
Llera has been signed as a replacement for Keith Andrews, who moved to Blackburn Rovers last week.
Di Matteo said at the time of Andrews' departure: "He is going to be a big loss for this club, he has been fantastic here and it is a bit of a shame.
"But it is the player's wish and dream and we wish him good luck.
"It is going to take time to see how much he'll be missed here and obviously you know the transfer window closes in a few days, so there is not much time left.
"I would have been happier if it hadn't dragged on for so long, it hasn't been an ideal situation I have to say, but we just get on with life."