Cleveland hold onto first place in the AL Central, thanks to a complete game win by Justin Masterson, who struck out nine while holding the visiting Reds to three hits in a 7-1 Indians' victory.
With the White Sox blanking the Cubs, 7-0, Cleveland maintained their 1/2 game lead in the division and completed the sweep of Cincinnati, who are still 2 1/2 games clear of Pittsburgh in the NL Central.
Montero's fourth-inning blast got the Diamondbacks to within one run of Seattle at 5-4, and they tied the score later in the frame. Scoring five in the fifth and four more in the sixth inning, the Diamondbacks pulled away from the visiting Mariners and ended their series taking two of three.
Arizona continues to struggle along in the NL West, a game under .500 at 34-35 and eight games behind the LA Dodgers.