Girardi told the pair of the change before Monday's series opener at Yankee Stadium after waiting to see if he would need Phelps as a long reliever over the weekend in Toronto.
"It's awesome. It's a big game," Phelps said. "We need these three coming home. To know they have faith in me going out there to give us a chance to win, it's a big deal."
It will be the 11th start of Phelps' career and first since Sept. 19, when he returned to the bullpen in the wake of Andy Pettitte's return from the disabled list.
Nova posted a 6.83 ERA in his past 10 starts on either side of a short DL stint with shoulder soreness, and he will move to the bullpen, according to Girardi.
"He's the manager and he's doing the best that's for the team," said Nova, who acknowledged he was disappointed and wants to start but isn't surprised by the decision given his lack of command. "That's what I think. If he thinks that's the best way, I'll go there."
Two of Phelps' 10 career starts came against the Red Sox, along with two relief appearances. The righty is 1-1 against Boston as a starter with a 2.92 ERA and a 3.44 ERA in all four appearances, and he believes his familiarity with the lineup will serve him well.
It will undoubtedly be the biggest start of the 25-year-old's career. When Phelps started an early September game against Baltimore, he admitted he felt the pressure of a pennant race early on and allowed four runs in the first inning before settling down.
"I want to pitch," Phelps said. "I want to pitch in big games, and they don't get a whole lot bigger than down the stretch in a pennant race."