First, there's the 3-0 advantage. This argument notes that the Dodgers easily swept the Cardinals, finishing their series on Saturday. In contrast, the Phillies needed another game and two more days to defeat the Wild Card team. Thus, the Dodgers are at an advantage because they are better physically rested, less emotionally spent, and showed their dominance by going 3-0.
Second, there's the lefty starter argument. This argument notes that the Phillies' power core of Chase Utley, Ryan Howard, and Raul Ibanez bat left-handed. The Rockies showed the Phillies a steady diet of right-handed starting pitchers. And their manager didn't even pull his right-handed closer facing Ryan Howard in the most important situation of the season. The Phillies obviously mash righties. But the Dodgers' starting rotation has Randy Wolf and Clayton Kershaw, both very good lefties. And their bullpen has Phillie-killer Hong-Chih Kuo (in 31 plate appearances, the Phillies have a .277 OPS against him) as well as George Sherrill. Thus, the Dodgers have an advantage.
Well, rest assured, the Phillies may be at a disadvantage against the Dodgers, but not for these two reasons. I'll address them in order.
The 3-0 advantage: This one is fairly simple to address. Since 1995, when the modern Wild Card system was instituted, there have been 16 Championship Series featuring a team that went 3-0 in the Division Series facing a team that didn't go 3-0 in the Division Series. Of those 16 Championship Series, the 3-0 team has won only 6 times. Thus, the team that was worse in the Division Series has won 62.5% of the time.
Of course, this doesn't mean the Phillies' chances of winning are 62.5%. That's not how statistics work nor is it what I'm saying. It's just clear from the historical evidence that the team that went 3-0 does not have an advantage because of that record in the first round of the playoffs. In fact, it's possible (although certainly not proven) there's a disadvantage of some sort.
The lefty-starter advantage: The Phillies certainly dominated the Rockies' righties. In the NLDS, they had an .867 OPS against Rockies' righties compared to just a .408 OPS against their lefties. But this is a tragically small sample size, on both fronts: 157 plate appearances against righties versus only 13 against lefties.
With a much bigger sample size, the Phillies' favoring righties over lefties disappears. In fact, it gets reversed slightly. This year, the Phillies had a .779 OPS against righties compared to a .787 OPS against lefties. Don't worry about sample size here. The Phillies had 4593 plate appearances against righties and 1745 against lefties. So let me state this again just to be clear: the Phillies hit better against lefties than righties. With Utley, Howard, and Ibanez, how's this possible?
Yes, the Phillies lefties do worse against lefthanded pitching, but only going from an .814 OPS against righties to a .793 OPS against lefties. The big difference is that the Phillies righties do much better against lefthanded pitching, going from a .717 OPS against righties to a .783 OPS against lefties.
And, measured against the rest of the league, the Phillies here are at a huge advantage. The Phillies' lefties sOPS+ (measure of how good the team's OPS is against the league for this split, with 100+ being better and below 100 being worse) is 108 against righties but goes up to 125 against lefties. The team's righties sOPS+ goes from 98 against righties to 102 against lefties.
So are the Phillies disadvantaged against the Dodgers in this matchup? Maybe so, but if they are, it has nothing to do with the Dodgers' sweeping the Cardinals or the Dodgers having very good left-handed pitching.