His two-run shot in the seventh inning gave Ken Cloude and the Mariners a 3-1 victory over the Boston Red Sox on Monday night.
"That was unbelievable," said Monahan, the grandson of NHL Hall of Famer Boom Boom Geoffrion. "I've been waiting for it. Better yet, we won."
Monahan, playing in his 20th game with the Mariners, hit a line drive over the right-field fence off John Wasdin (5-4) to break a 1-1 tie. Wasdin hit Jay Buhner with a pitch to open the inning.
"It takes a while to think you belong," said Monahan, 23. "In my mind, I still have to prove I deserve to be here."
"He's done a nice job here," manager Lou Piniella said of Monahan, called up from Triple-A Tacoma after the All-Star break when the Mariners put Glenallen Hill on waivers.
"He's played very well in the outfield and we're going to be patient with his hitting," Piniella added.
Wasdin was disgusted with himself for letting Monahan beat him.
"I worked so hard against getting the Griffeys and the Edgars and the Jays and the Rodriguezes out," he said, referring to Seattle's Ken Griffey Jr., Edgar Martinez, Alex Rodriguez and Buhner.
"It was a changeup right down the middle. Just mislocation," Wasdin added.
Cloude (6-7) won his third straight decision. He gave up five hits in eight innings, striking out three and walking three.
"He pitched a great ballgame," said Boston's Mo Vaughn, who went 0-for-4 and struck out twice against Cloude. "He deserved to win."
After Cloude walked Darren Lewis to lead off the eighth, Piniella came out to talk to him.
"I knew what he was going to say," Cloude said. "And I knew what I was going to say, `There's no way I was going to come out of this game.' I told him I had these guys, he left me in there and things worked out."
Mike Timlin pitched the ninth for his seventh save.
The Red Sox lost for the second time in seven games on their West Coast road trip.
Wasdin allowed eight hits and three runs in seven innings.
The Red Sox took a 1-0 lead in the fourth when Troy O'Leary homered off the facing of the third deck in right field.
The Mariners tied it in the bottom half on David Segui's single, Buhner's ground-rule double that hopped the fence in left-center field and Russ Davis' sacrifice fly.
Griffey Jr., who leads the AL with 41 homers, went 1-for-4 and did not homer for his fourth game in a row. Griffey has hit two homers in 17 games since hitting two on July 14.
The Yankees beat Oakland 14-1 on Monday night.
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