How is the team 8 games better?
I was looking at the difference in their W-L records, to be honest, just to keep it simple. Sale: 14-3, Pivetta, 5-8. 9 game difference in the win column, 5 game difference in the loss column = 7 games difference. So off by one. But that's not really how it should work so let's do a deeper dive.
In games when Pivetta has started, the Sox are 10-9. In those 10 wins, here's the IP and R allowed by Pivetta:
7.0 ip, 0 r
7.0 ip, 0 r
7.0 ip, 3 r
6.2 ip, 3 r
6.0 ip, 1 r
5.2 ip, 2 r
5.0 ip, 0 r
4.2 ip, 2 r
4.1 ip, 3 r
4.0 ip, 4 r
In those 9 losses, here's the IP and R allowed by Pivetta:
6.0 ip, 1 r
3.1 ip, 5 r
6.2 ip, 4 r
6.0 ip, 0 r
5.0 ip, 3 r
4.0 ip, 5 r
5.1 ip, 2 r
4.0 ip, 5 r
2.2 ip, 7 r
Chris Sale has only had 2 games all year where he's given up more than 3 R, and only 4 games all year where he's given up more than 2 R. In 19 games, he's given up 2 R or fewer. Pivetta has had 5 games where he's given up more than 3 R, and 10 games where he's given up more than 2 R. Plus, Sale has averaged 6 IP per start, while Pivetta has averaged fewer than 5.1 ip per start. So those extra 2 outs per game on average really do add up, helping the bullpen not only for that game, but for the games surrounding Sale's starts.
So maybe 8 was a bit much, but I have no doubt that if the Sox had THIS Chris Sale this year, they'd be comfortably in the playoffs right now if the season were to end today, and they'd absolutely have a shot against anyone once in the playoffs.
So you never know how these games would have played out, of course, so I'm just guessing, but he's pitched way better than Pivetta