Why? he was in the NL and his peripherals suggest he will see an increase in ERA.
I don't know that the peripherals really suggest that he's much worse than about a 3.50 ERA.
He's a heavy ground-baller who keeps the ball in the park. FIP uses actual HR/FB rate for the pitcher. xFIP uses expected HRs allowed based on the numbers of flyballs the guy gives up and the league average rate. Webb has a history of keeping the ball in the park at a really good rate, so I think FIP is probably the better indicator of what to expect from him instead of xFIP. His career FIP is 3.45 and last few years it's been 3.60, 3.26, and 3.62.
Plus, I'd expect our infield defense to help him out as well, so his ERA should be lower than his fielding-independent ERA.
He's still nothing special. I'd expect around a 3.25-3.50 ERA from him. League average for all AL relievers last year was 4.29, so he's certainly an above average reliever if he can do that, but he's not likely to be much better than our 3rd or 4th best arm out there.
The velocity drop you mention is a potential red flag, though.