An (dis)honorable mention should go to Justin Lawrence for how acutely terrible he was during his seven-game, six-inning stint that really highlighted the downside of trying to MacGyver a bullpen together on the fly
It's even worse when you consider that he was sorta getting acceptable results in his first three outings: 2.2 IP, 0 runs allowed, 1 hit, 4 K. Sure, he handed out three free passes in one outing and allowed two inherited runners (which turned out to be the winning runs) to score in another, but hey - he was available for a reason.
But in his last 4 outings: 3.1 IP, 12(!) earned runs allowed on 12 hits, including 4 homers and 6 walks, plus an additional inherited runner allowed to score for good measure.
He obviously didn't play enough to be LVP, which requires threading the needle of being bad but not so bad that you get cut or benched. But he was the exact opposite of bang for your buck - they even signed up to pick up his full non-minimum salary. Luckily the Nationals were kind enough to take him and his salary commitment off their hands