Steve Carlton appeared to have ended a slump after pitching a four-hitter for eight innings, but the question everyone was asking was: what's wrong with Tom Seaver?

With Carlton pitching well and Bo Diaz driving in three runs, the Philadelphia Phillies easily defeated the Cincinnati Reds, 9-1, last night in Riverfront Stadium. Seaver, 1-6 with a 6.86 earned run average, yielded nine hits in five innings. He said the poor start puzzles him.

"I've been pitching poorly for almost two months," Seaver said. "I don't think I've ever pitched this badly for this long of a time."

Seaver said he is healthy after battling a pulled thigh muscle and an upper respiratory virus this spring, but Phillies third baseman Pete Rose isn't so sure.

"I think there was something wrong with him, to be honest," Rose said. "You could see wincing in his face when he threw. He'd make a lot of faces when he threw, like something was bothering him. I hope it's not serious . . . he wasn't Tom Seaver."

Carlton (5-6), beating Cincinnati for the first time since May 1980, struck out three and walked three before being replaced by Warren Brusstar in the ninth.

Philadelphia scored three runs in the second inning, and Diaz hit his ninth home run in the fifth.