The current Sox' bullpen looks like this:
Ottavino - 3.27 era, 1.36 whip, 10.2 k/9
Taylor - 3.44 era, 1.41 whip, 10.5 k/9
Workman - 3.71 era, 1.82 whip, 6.4 k/9
Valdez* - 3.45 era, 1.26 whip, 9.4 k/9
Whitlock - 1.31 era, 1.04 whip, 9.9 k/9
Hernandez - 3.55 era, 1.52 whip, 12.3 k/9
Barnes - 2.36 era, 0.83 whip, 13.9 k/9
Rios - 3.60 era, 1.40 whip, 6.6 k/9
Sawamura* - 2.87 era, 1.25 whip, 10.5 k/9
*Valdez replaced Sawamura when Sawamura went on the IL.
Now they also have Houck available if they want him in the bullpen, especially in the playoffs. And they have Bazardo who might be as good as he looked in his brief stint with the MLB club earlier in the year (3.0 ip, 0.00 era, 1.00 whip, 9.0 k/9).
But of this group, the most likely to be replaced are, in order (IMO):
1. Rios. High whip, low k/9.
2. Workman. Even higher whip and lower k/9, but has earned some trust because he's done it on the big stage for this club before.
3. Valdez. He's been good but I just don't know that there's full confidence in him. His last couple of outings have been amazing though.
4. Sawamura. He's been steady all year long but I think he is next on the list.
5. Hernandez. Can be totally dominant, but he allows too many baserunners to make me feel comfortable.
6. Taylor. After a rough initial outing this year, he's been nails. Barring injury or collapse, he and his 95 mph fastball from the left side aren't going anywhere.
7. Whitlock, Ottavino, Barnes. None of them are remotely close to being let go. They've all been superb.
So Rios, Workman, and Valdez are positions that could use upgrading, even though they've all been solid for Boston. Honestly (knock on wood), this is by far the best overall bullpen I've ever seen the Sox have. Several totally dominant pitchers, but also a ton of quality up and down the bullpen. And they could add Houck (2.50 era, 1.17 whip, 11.5 k/9) to the pen as well if they want. But imagine replacing Rios or Workman with Craig "I'm back to being dominant" Kimbrel and his 0.49 era, 0.71 whip, and 15.7 k/9.