Santiago received the loss after allowing five runs on four hits over 2 2/3 innings. He also struck out four and walked three.
Santiago is winless in his last eight starts.
Doubront allowed four runs on seven hits over six innings of work. He also struck out five, but walked four. He is now 2-1 with a 3.99 ERA.
Monday was his seventh start of the season and first win since July 7.
Lewis (14-7) received the loss after allowing six runs (five earned) over 5 2/3 innings of work. He also struck out two and walked two. His ERA is now 4.50.
Lewis has now lost his last two outings with no quality starts.
Keuchel (16-6, 2.24 ERA) allowed a solo home run to Mark Trumbo in the second inning for the only damage against him. Otherwise, Keuchel as flat-out dominant against the Mariners.
The Cy Young Award candidate was absolutely brilliant in August. In six starts, he went 4-1 with a 1.95 ERA and a 41:8 K:BB ratio.
Nova (5-7, 4.50 ERA) was done in by the longball in this one. He allowed a two-run home run to Mookie Betts in the third inning, followed by a solo shot to David Ortiz in the fourth for the only damge done against Nova.
This was a good bounceback effort for Nova, who allowed seven runs in his last outing. Overall, Nova had a rough month of August. In six starts, he went 2-4 with a 5.68 ERA.
Rodriguez (8-5, 4.25 ERA) gave up a run in the first inning, thanks in-part to a fielding error. Another run came across in the fourth, but the Red Sox scored three runs of their own in between. The bullpen made things interesting, but ultimately the Red Sox held on for the win.
The young right-hander pitched well in August, going 2-2 with a 4.06 ERA--which includes an eight-run meltdown back on August 12.
With Bassitt out Tuesday, the A's will turn to reliever Cody Martin to start the contest. Martin is 2-3 with a 5.40 ERA over 21 2/3 innings.
Chen allowed an early run in the second inning, but that was just the beginning. He served up a two-run home run to Asdrubal Cabrera in the fourth before giving up two more runs in the fifth, including a solo shot by Evan Longoria.
This was the first time in the month of August that Chen allowed more than three runs in a start. Overall, in six starts this month, Chen went 3-1 with a 3.79 ERA.
Archer struggled (by his standards) in his last outing, allowing four runs in six innings. But overall, he finishes August 3-2 with a 3.66 ERA in six starts.
Gonzalez allowed four runs on seven hits over 5 1/3 innings in Sunday's start.