Lannan signed a $2.5 million, one-year contract on Tuesday, adding depth to a rotation that lost starter Vance Worley when he was traded away.
The 28-year-old Lannan was 4-1 with a 4.13 ERA in six starts for the NL East-champion Washington Nationals this year. He also made 24 starts for Triple-A Syracuse, going 9-11 with a 4.30 ERA.
Lannan struggled against the Phillies over the years. He was 3-13 with a 5.53 ERA against them, including a 6.49 ERA in eight career starts at Citizens Bank Park.
"I think it's more me not executing," Lannan said. "That's what it comes down to, and it's against really good Philly lineups. So I think it comes down to me executing pitches. It doesn't really matter where you're pitching. This park is smaller. You've just got to execute your pitches."
Lannan's deal includes performance incentives. He gets $200,000 each for 150, 160, 170, 180 and 190 innings, $250,000 each for 200 and 210 innings and $250,000 for 31-34 starts.
Overall, Lannan was 42-52 with a 4.01 ERA in 134 games -- all starts -- in six seasons for Washington. Worley was dealt to Minnesota along with minor-league righty Trevor May for center fielder Ben Revere.
"It's a great opportunity," Lannan said. "I feel like the Phillies have a great shot of winning the NL East and going to the World Series."
After finalizing the deal for Lannan, the Phillies introduced new third baseman Michael Young. The seven-time All-Star arrived in a trade with Texas earlier this month.
"I couldn't be more excited to be a member of this organization right now," Young said. "This is a team with a really strong history of winning over here the past five or six seasons. Great history and a great fan base. A loud, intense and fun ballpark. All those things really appealed to me."
A former AL Gold Glove winner at shortstop, Young hasn't played third base regularly since 2010. He made 40 starts at first base, 25 at third base, 14 at second base and four at shortstop last year. Young feels he will benefit from concentrating on one position.
"My time at third will be easier because I can donate all my reps to third," he said. "The toughest part of playing different spots is dividing your reps. Taking 25 percent of your balls at third because you have to go to short, second and first. I'm looking forward to that and getting down to Clearwater and getting a lot of reps under my belt."
Young batted .277 with eight homers and 67 RBIs in 2012, a down year for him. From 2003-11, Young hit at least .300 seven times and averaged 17 homers and 90 RBIs.