Right after Josh Fuentes singled and advanced to third on a Marcus Semien throwing error, Diaz lined a hanging curveball just over the left field fence in the corner for a 2-1 Rockies lead.
It was Diaz' first home run and first RBIs this season. Semien's error was the first in nine games for the A's, who entered Tuesday with 20 errors, fewest in Major League Baseball.
Manaea was solid Tuesday but not quite as dazzling as he had been in recent starts.
Instead of a fastball in the 95 mph range, Manaea's four-seamer was more around 91-92 mph.
Despite some decreased velocity on his fastball, Manaea's outing was still solid. He threw 89 pitches, finished with season-high seven strikeouts and was charged with three earned runs over 5 2/3 innings.
Manaea came into Tuesday with a 4-0 record and a 1.98 ERA over his last five starts. In his last outing - in Oakland against Houston on Sept. 10 - Manaea allowed just one earned run and two hits over seven innings in a 3-1 A's win.
At least the A's schedule lets up a bit going forward. They finish their series with the Rockies on Wednesday afternoon, fly home, have a day off Thursday and begin a three-game series with the Giants on Friday at the Coliseum.
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