The rookie lefty will start Wednesday in place of struggling righty Kyle Kendrick in the middle game of a three-game series in Atlanta. He'll oppose fellow rookie Jair Jurrjens.
Kendrick has struggled, posting an 11.56 ERA in his previous six outings, and the 25-year-old Happ has appeared in five games for the Phillies. In two starts, he has posted a 3.27 ERA.
Strategically, the Phillies went with Happ because of Atlanta's unfamiliarity with him, as well as the presence of lefties Brian McCann, Casey Kotchman, Kelly Johnson and Gregor Blanco.
"We've got a better matchup," Dubee said.
Meanwhile, Kendrick needs a break, mentally and physically.
"He's pitched so much," Dubee said. "Part of it is mental, and part of it is he's been beaten around a little bit. He has to understand you have to develop a repertoire. In this league, he's no longer a foreign guy. Everybody has seen him now, some teams multiple times. He can't sneak up on them anymore. He has to be willing to make adjustments because these guys are adjusting to you."
Dubee hopes that will happen in the offseason. He wants Kendrick to attend instructional league to gain more confidence in a changeup and other pitches. The right-hander has relied too much on his sinker.
"[He needs to] improve his slider, add a changeup, one that he can use and is willing to use," Dubee said. "He's not going to get by on one pitch. Hitting isn't easy, but when you simplify it, it becomes a little easier."
Kendrick said last week he agrees with Dubee's assessment, and understands he'll have plenty to do in the offseason. He declined to comment on Tuesday.
"I still think I can get big league hitters out, but those pitches will help make me better," Kendrick said last week. "I want to do better and want that pitch. After the season, I'll relax a little bit, and when I start throwing again, I'll head to Spring Training early and get better, keep throwing."
Until then, Happ will get his shot in a key spot. Manuel and Dubee said his performance could determine whether he starts Monday, the final time the Phillies would need a fifth starter.
Happ said he will try and curb his nerves.
"It's kind of one of those things where you get butterflies until you get that first hitter," Happ said. "I'm going to try to use it to my advantage. I had some time and feel like I'm unemotional out there, the way it is. I'll just try to go with the way I've been throwing."