He's looked capable of regaining that status, but with the San Diego Padres.
Kennedy goes for another strong outing Tuesday night when he faces the Diamondbacks for the first time since they dealt him to the Padres.
Kennedy (5-9, 4.88 ERA) went 21-4 with a career-low 2.88 ERA in 2011, tying for the NL lead in wins while helping Arizona to a division title and its most recent playoff appearance.
The right-hander failed to remain that effective for the Diamondbacks, going 18-20 with a 4.47 ERA over the next season and a half before being sent to San Diego prior to the non-waiver trade deadline July 31.
Kennedy is 2-1 with a 3.20 ERA in four games with the Padres (59-72), turning in his best performance Wednesday to beat Pittsburgh. He tossed seven shutout innings while striking out eight in a 2-1 win.
"We're seeing the pitcher we thought we'd get," manager Bud Black told the team's official website.
Now, Kennedy will take the mound at Chase Field for the first time as a visitor.
"It's going to be emotional for him," Black said. "It's a different feel. I've been there. I think that it's a unique feeling."
Black's club has been struggling offensively, scoring 18 runs over the past seven games while hitting .143 with runners in scoring position. The Padres fell 6-1 on Monday in the opener of this three-game series as they mustered five hits - all singles, and one came from starting pitcher Tyson Ross.
Will Venable had two of those hits. The veteran outfielder is among the NL's top hitters this month with a .374 average in 22 games, getting at least one hit in 21 of them.
He's 9 for 25 (.360) in 13 season meetings with Arizona (67-63).
The Diamondbacks have won seven of nine at home, including four in a row. They haven't had a longer winning streak in Arizona since an eight-game run July 6-24, 2012.
Paul Goldschmidt, who has a career-high 31 homers, is hitting .349 with 10 RBIs in the last 10 games after connecting for a two-run triple Monday. He's the first Diamondbacks player to amass 103 RBIs since Luis Gonzalez had 104 in 2003.
Martin Prado is looking to continue his own impressive production, hitting .365 with 27 RBIs in his last 23 games. He's driven in eight runs over the past three contests, including three in the series opener.
"I'm taking advantage of the situation," Prado said. "I had a difficult course over the first few months and it's good to get going this way."
Trevor Cahill was scheduled to get the ball Tuesday, but he tossed the final four innings of Saturday's 12-7 win in 18 at Philadelphia.
The Diamondbacks are now expected to send David Holmberg to the mound for his major league debut in a spot start.
The 22-year-old left-hander is considered one of the team's top pitching prospects. He's 5-7 with a 2.66 ERA in 25 starts for Double-A Mobile this year, including a 1.59 ERA over his last six.