Ask any informed fan about the Phillies' woes based on the schedule, and you're likely to get two answers: 1) the team sucks out of the gate, and 2) the team really suffers during interleague play. Looking at the team's win/loss record over the past three years based on month shows this perception is not entirely accurate, but is pretty close:
The Phillies have an overall winning percentage of .564 over the past three years. The only two months they're below that figure is March/April when they have a .504 winning percentage and June when it's at .475. In fact, without those two months, the Phillies have won almost 60% of their games (.598 winning percentage).
So, will Roy Halladay help with these two maladies? The answer is split, as he should help in June, but April might continue to be a problem.
Let's start at the beginning of the year. Historically, in April, Halladay is at his worst. Over the course of his career, he's appeared in 55 March/April games, starting 52 of them. He has slightly lower, but roughly comparable, numbers of appearances/starts in the other months as well.
Almost across the board, Halladay has his worst or second-to-worst career marks in April. His career April ERA is 4.13, his worst by month. His RA is 4.55, also his worst. His WHIP is 1.242, tied for second worst. His strikeout rate is 6.4, tied for worst. His opponents' OPS is .717, second worst. And in each of his last two years, Halladay has had his second worst monthly ERA in April.
Of course, none of this is to say that he won't have a stellar month of April this year. After all, in 2007, his April ERA was his best for the year. And this analysis doesn't mean that, even if not his best, this April from Halladay won't be better than someone else that the Phillies ran out there in years past. However, based on his past, don't look for Halladay to be an immediate savior for the team that struggles out of the gate.
On the other hand, Halladay may just help in June. Overall, June is a better month for him. He has a 3.57 ERA, 1.176 WHIP, and .667 OPS against.
But, more importantly, the Phillies' June struggles have come in large part from being awful in interleague play. Obviously, Halladay's dominance over the past decade has come against AL teams. Although he's actually had more success in his career against NL teams (3.02 ERA in interleague play compared to 3.48 ERA against the AL), he still has had huge success against the AL.
This analysis does not change that Halladay is going to be a huge asset for the Phillies this year. However, to the extent there's some big-picture institutional issue going on with the team at the start of the season, don't expect him to radically alter the team's problems early on. But, you can expect him to help a team that's historically struggled against the AL.