This is going to be short and sweet.
Duffy started the season in the rotation; he made eight starts before going on the DL. In those eight starts, Duffy was pretty miserable. He had only a 16.9 percent strikeout rate and a whopping 10.7 percent walk rate. His luck stats, BABIP and LOB%, indicated some bad luck, but Duffy was allowing a lot of good contact. His line drive rate was really high (27.4 percent), and his hard contact rate allowed was 36.2 percent (MLB average is 28.5 percent). What this added up to was a 5.87 ERA / 4.53 FIP / 4.79 xFIP in 38 and 1/3 innings (only 4.8 innings per start).
Then Duffy went on the disabled list.
Conveniently, Duffy has made eight starts since returning from the DL (including last night's). He has thrown 50.2 innings in those eight starts (6.1 innings per start). Duffy has eschewed strikeouts even more; his strikeout rate of 11.9 percent is almost equal to Jeremy Guthrie's. His walk rate of 8.9 percent is a little better, so that's good. Importantly, Duffy has allowed slightly less solid contact - a 24.2 percent line drive rate and 32.3 percent rate of hard contact. His popup rate is very high.
There's a problem though.
His luck stats, BABIP and LOB%, have swung completely in the other direction. His BABIP allowed is far below MLB average at .243, and his LOB% is far above MLB average at 86.1 percent. What this has led to is an overall line of 2.66 ERA / 4.87 FIP / 4.95 xFIP. Duffy's underlying indicators don't show improvement, but he hasn't allowed too many runs. The weaker contact could be part of that.
Notably, Duffy's velocity has fallen throughout the season. His average four-seam fastball velocity in April was about 95mph. In his start last night, which comprises the entirety of his August average four-seam fastball velocity, Duffy averaged 93.3mph on the same pitch. His sinker and slider have seen similar velocity decreases, but those pitches have a smaller sample size. Velocity decreases aren't good for performance, although the interplay between velocity and command could be an issue for Duffy (as it could be for Yordano Ventura). Can Duffy command the fastball better at 93 compared to 95? I don't know the answer to that.
Duffy has, however, done a better job of keeping his fastball up. Duffy's fastball is within a class of "riseballs" - it does not sink nearly as much as other pitchers' four-seam fastballs. It's a popup pitch. He got only five popups with the pitch in his first eight starts; he has 11 popups on the pitch after returning from the DL. He's also done a better job of getting whiffs with his four-seamer after returning from the DL compared to before.
As far as future performance, it's tough to say anything. It's not a large sample size either before or after the DL. If Duffy can keep generating weak(er) contact while stabilizing his strikeout and walk rates, he'll likely return to something approximating last year's performance. If not, well, Duffy's performance will look very similar to Guthrie's.