They need the win to avoid a three-game sweep at the hands of the Blue Jays, who are dawdling in fourth place in the American League East.
More important, they must reverse a recent trend that threatens their playoff hopes.
They let a 5-0 lead get away Saturday afternoon and lost 8-6 to the Blue Jays on a three-run double in the bottom of the eighth inning by Aledmys Diaz.
The Phillies, who entered Saturday three games behind the first-place Atlanta Braves in the National League East, have lost six of their past seven games.
"We're still in a good position to strike," Phillies manager Gabe Kapler said.
The Blue Jays have won five in a row for the first time this season and hold a 4-1 lead in the season series against the Phillies.
The Phillies (69-60) will start right-hander Vince Velasquez (8-9, 4.06 ERA) Sunday and the Blue Jays (60-69) will counter with right-hander Marco Estrada (7-9, 4.88).
Velasquez has never faced the Blue Jays in the regular season, although he has pitched against them in spring training.
Estrada has not pitched against the Phillies in the regular season this year, but in 11 career games (five starts) against them he is 2-1 with a 5.45 ERA.
The Phillies have struggled on the road this season, going 28-48 (41-21 at home) and 5-14 in their past 19 games.
"I think we maintain the same poise that we have in rough stretches all season," Kapler said. "I think we've had stretches similar to this one. This one has been undoubtedly difficult. but we want to remind our guys how tough we are and how resilient we are, how teams have rough stretches and this by no means does this mean we don't have the ability to win the National League East.
"I think one of the things that we can focus on is the reason this one may be a little bit more challenging is because it takes more energy to be resilient when you're closer to the end. So it just means that we're going to need to dig real deep and get that last drop of energy. I believe we have it in us."
While Lourdes Gurriel Jr. was 0-for-3 with a walk Saturday to end his hitting streak at 12 games, the home run streak of Kendrys Morales is alive. He has hit home runs in six consecutive games (for a total of seven) that matches the club record set in 2001 by Jose Cruz Jr.
Morales also has a nine-game hitting streak. His batting is .516 (16-for-31) with seven homers and 13 RBIs.
"He's been on a nice little roll now for a while, a good while," Blue Jays manager John Gibbons said. "Early in the year, he was chasing a lot of breaking balls, off-speed pitches down in the dirt, down in the zone -- that's what was getting him into trouble. He's laying off those now and he's even hitting breaking balls."
Outfielder Billy McKinney hit a two-run homer Saturday and has three homers in his first seven games from the Blue Jays after being called up from Triple-A Buffalo. He was part of the deal that sent left-hander J.A. Happ to the New York Yankees on July 26.
McKinney is the first player to combine three homers and nine RBIs in his first seven games with the Blue Jays. Seven others have hit three or more homers in their first seven games with Toronto and Dave Martinez had nine RBIs in his first seven games with the Blue Jays in 2000.