Wells lost for the first time in almost two months while Chen threw five scoreless innings Saturday as the Phillies beat the Toronto Blue Jays 7-3.
Chen (5-0), acquired from Atlanta along with lefty Jim Osting for Andy Ashby on Wednesday, allowed just three hits in his first start since last Oct. 2.
"David's got the best record in baseball, so this was huge for me," Chen said.
The 23-year-old left-hander, on a strict pitch count, struck out three and walked none. He threw 66 pitches, 42 for strikes.
"Atlanta has some great pitchers, so the only place for me was the bullpen," he said. "I've a good opportunity to start here. It's a fresh start."
Wells (15-3), who started for the AL in the All-Star game on Tuesday night, had won his previous eight decisions. His last loss came on May 20 against the Chicago White Sox.
"You're not going to win every game," Wells said. "They wore me out. They were on every pitch."
Wells gave up six runs against the Phillies, tying a season high. He allowed eight hits in seven innings.
"Anytime you can score runs off a guy like that, it's huge," said Ron Gant, who hit a two-run opposite field homer off Wells in the seventh for a 6-3 lead. "His record and his poise speaks for itself."
Scott Rolen had three his, including a solo home run in the second and Gant drove in three runs.
Rolen is 4-for-6 with two homers lifetime against Wells.
"I've had some success against Wells, but he's not a guy I want to face everyday," Rolen said.
Gant's sacrifice fly in the fifth made it 4-0.
The Blue Jays rallied for three runs in the sixth off reliever Steve Schrenk. After Craig Grebeck walked and Raul Mondesi doubled, Carlos Delgado hit an RBI grounder. Tony Batista followed with a two-run homer, his 27th.
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