The Indians put it away with six runs in the ninth, the first five off Danny Graves. The closer was booed off the field and then yelled back at a fan near the dugout and made a gesture with his hand.
"I don't know what he said," said Graves, who has been verbally abused by fans for saying that they shouldn't boo. "It's not an issue. If they want to cuss at me, fine. One thing I said two weeks ago got blown out of proportion, and now it's way out of proportion."
Cincinnati has lost 18 of its last 23, falling a season-high 13 games under .500. The Reds haven't won any of their last 11 series -- their longest such slump since Pete Rose's gambling scandal in 1989 -- and haven't even won back-to-back games in more than a month.
"Nothing's coming together," said Reds starter Ramon Ortiz (1-3), who left in the sixth inning after a grounder deflected off the left side of his face. "We're losing games -- unbelievable. I want to see everybody happy."
The Indians improved to 19-23 but remain 11 games behind the White Sox in the AL Central. Ten of their next 13 games are against division rivals Minnesota and Chicago, providing a chance to make up ground.
"I think it's more important for us to get ourselves going, work off winning this series, look and see what we've done," manager Eric Wedge said. "We've still got a long way to go. We need to be more consistent offensively."
Sizemore, one of only three batters in the Indians' lineup hitting above .260, tripled home a run in the third and scored on Casey Blake's single off Ortiz, who was part of the Reds' failed offseason attempt to upgrade their rotation.
Sizemore also singled home a run in the ninth off Graves, who retired only one of his six batters. Blake had a two-run double and Ben Broussard's RBI single finished Graves, whose ERA rose to 7.36.
"I'm not worried about losing my job or being released," said Graves, the team's career saves leader. "I'm just worried about going out there and trying to do what I've done for eight years. Accountability factor? If I'm not doing what I'm supposed to do, that's their decision."