Going by Spring Training results, the answer appears to be Alfredo Aceves (last Saturday's start against the Phillies notwithstanding). Bard seems to have more trouble in the command department, channelling Dice-K and Miller in his most recent starts. The AL East isn't getting any easier and teams need deep rotations to compete. It's a big gamble to send Bard out there to give up a handful of runs (and walks) every 5th day, putting pressure on both the offense and the rest of the starting rotation. If Aceves, as valuable as he is to the bullpen, can command his stuff, keep walks and runs to a minimum, and pitch deeper into games, he should get that last spot.
What of Aaron Cook, who, in his limited ST outings has shown he deserves a shot to start? Can he get a few starts under his belt in AAA to get stretched out, then take over the fifth spot? While the sample size is extremely tiny, he looks to be healthy and he has a May 1 opt-out date. If he can nail down a spot in the rotation, it can only help strengthen the bullpen, with both Bard and Aceves added to the mix.
Then there's Dice-K, who could be ready to return to the big club as early as June. Will having Bobby V, who speaks some Japanese, help him become more like the pitcher he was in Japan? If he can come back strong from TJ surgery, he could really give the rotation a boost. A fresh start with a new manager and pitching coach could be the best thing for him.
Whatever the back of the rotation ends up being, having depth is the key to avoiding another September meltdown.