Consider how terrible Lucas Harrell has been this season:
6.01 ERA/1.74 WHIP. 81K:82BB. 19 homers allowed. .858 OPS-against (.390 OBP-against).
9.19 ERA/2.10 WHIP since the All-Star Break with 14K:25BB in 32.1IP.
ERAs over 7.00 in May (7.53), July (8.41), August (11.08), and September (9.82).
In the last week, he's allowed 14H/8ER, 5K:5BB, 2HR in 7.1IP.
But look specifically at Oakland, and they're responsible for a decent chunk of Harrell's problems. It started in G6 at home: 4.1IP, 7H/8ER, 2K:5BB, 3HR.
In G49 Harrell lasted 1.2IP, allowing 6H/6ER, 1K:3BB.
There were back-to-back relief stints in G119/120 where he held the A's to 2H/0ER, 1K:1BB in 4.1IP.
And then yesterday, when Harrell allowed 7H/7ER, 2K:2BB in 3IP.
So in five games total against the As, Harrell has thrown 13.1IP, 22H/21ER, 6K:11B, 4HR for a 14.18 ERA/2.48 WHIP. And to narrow that down to the three starts, it's 9IP, 20H/21ER, 5K:10BB.
And before you go saying "Well, it's just Oakland. They're good" (you'd be right), but keep in mind that, against Texas, he has a 10.05 ERA/2.23 WHIP in four appearances. Detroit? Two starts, 12.54 ERA/2.57 WHIP; St. Louis? Two appearances, 12.79 ERA/2.68 WHIP.
I don't know what you take from this. But it's clear that good teams who see Harrell more than once can take advantage of it.