A couple of years ago now, one of my colleagues said during a podcast "You can never have too many centre backs" and Villarreal has continually proved the wisdom of that statement. Now it's Eric Bailly's turn to go down, his shoulder injury, which he's had for a couple of months now, isn't responding to treatment and his performance against Espanyol indicated it's affecting his work on the pitch. Physicality is a major part of his game and if he is afraid to go into a challenge, that's not good.
The plan is for him to rest the shoulder and see if it's better after that time, or he requires surgery. Marcelino suggested Villarreal might be looking for a centerback in the transfer window, as otherwise we'd have to rely on Mario Gaspar or Bruno Soriano as an emergency fourth center back if we need them-- not good as it takes two players away from their best positions.
Otherwise, expect to see Ruiz and Musacchio as the starting pairing, with Daniele Bonera as backup. He played quite well in the second half versus Espanyol. It's doubtful we'd call someone up from the B team given we really want those guys to stay together and get promoted.