One thing to keep in mind, it has been unusually difficult to pitch for the Red Sox this year. Everybody knows how bad the defense has been this year, but Fenway has also been a strong hitter's park-- 109 park factor for hitters, 108 for pitchers this year. That's a rough combination for pitchers.
Our pitchers have allowed a ridiculous .320 BABIP at Fenway this year. Playing in a good hitter's park with lousy defense has to contribute to that in a big way. (On the road, it's .299.)
Sox pitchers have a 4.85 ERA at home this year, compared to 4.38 on the road.
Any pitchers we bring in will have to contend with Fenway, and while we hope the defense will improve, it's not likely to be very good overall next year.
This year, because the starters have been pulled early so often, the pitching has been absolutely battered in the 4th, 5th and 6th innings. The starters frequently get knocked out in the 4th and 5th, and then are followed by the worst of the bullpen.
ERA by inning:
1: 4.47
2: 4.59
3: 4.41
4: 5.29
5: 5.16
6: 6.23
7: 3.65
8: 3.90
9: 3.63
OPS allowed by number of pitches thrown:
1-25: .770
26-50: .725
51-75:
.855
76-100: .776
Obviously bringing in 2 starters who routinely pitch into the 6th inning effectively would be huge. But those guys are rare, and hard to acquire. Especially when you need 2 of them and you need them to them to do that at Fenway with a lousy defense behind them. Even a prototypical "innings eater" who isn't that great overall could help a lot. But figuring out who could do that for this team, with this defense and ballpark, is the hard part.
Quality starts is not a great stat but it's fine for a quick and dirty look at who pitches effectively through 6 innings the most often:
https://www.espn.com/mlb/stats/player/_/view/pitching/table/pitching/sort/qualityStarts/dir/desc
Bello leads the Red Sox with 14 QS in 25 starts this year, which is tied for 27th. Paxton is next with 8 in 19 starts, tied for 75th.
The Red Sox are 26th in baseball with 38 QS out of 143, ahead of only the Tigers, Rockies, Royals and A's. If that doesn't improve dramatically next year, the pitching will suck again. Being near the top of the QS standings doesn't mean much, but if you're down near the bottom, then you are likely to be a bad team.
It's actually pretty surprising the Red Sox have a winning record (so far) with only 38 QS. (All the openers we've used helped in that sense.)