The idea is that these catchers were temporarily replaced. Catcher is a position for which it is probably very hard to simply move a guy from another position. In the outfield, you can move a right fielder to center, for example. Maybe move a SS to 2B or a 3B man to 1B. But at catcher, you really need to find another guy who is a catcher.
Unfortunately, there were not many cases. Only 11. And I used two seasons for Fisk, 1974 and 1975. He came back to being a regular in 1977. In almost all cases, the catcher had 400+ PAs in both season 1 and season 3.
Then I found the difference in PAs for each player, comparing season 1 to season 2, and season 2 to season 3. A catcher might have 500 PAs in season 1, then 200 in season 2 and 450 in season 3. So in one case I could assign 300 PAs to the "replacement" catcher and 250 in the other.
That means I did two calculations for the OPS+ of the "replacement," one using the difference in PAs between season 1 and season 2 and the other using the difference in PAs between season 2 and season 3. The "replacement" OPS+ is a weighted average of the OPS+ of the X number of PAs for catchers on the team in season 2 who ranked lowest in OPS+.
For example, if a team needed to replace 300 PAs at catcher, I ranked any catchers on the team from highest to lowest in OPS+. Then I came up with a total number of PAs that equaled 300 using the lowest ranked catchers in OPS+ on the team. It might be 100 PAs from one catcher and then 200 from another. Then I calculated their combined OPS+, weighted by PAs.
The table below shows the OPS+ for the replacements. ROPS+1 refers to the OPS+ of the "replacement" assuming the number of PAs needing to be replaced is the difference in the regular starting catcher's PAs from season 1 to season 2. ROPS+2 is for the season 2-3 difference. The last column shows the OPS+ for the regular starter over the three seasons in question.
Generally, the replacements did much worse. ROPS+1 has a weighted average of 64 while ROPS+2 has a weighted average of 60. The weighted average of the OPS+ for all these catchers, over the three seasons (4 for Fisk) is 109. All catchers with 1000+ career PAs since 1945 had a combined OPS+ of 92 (also a weighted average).
So when teams have had to "replace" their regular starting catcher, temporarily, the "replacements" had an OPS+ in the range of 60-64 while an average catcher would have had 92. But the drop off compared to the actual starters on these specific teams was even greater, since they a had a combined OPS+ of 109.
Why the guys who needed to be replaced tended to be above average is not clear. I did look at the guys with the longest careers and maybe one of the reasons why they had long careers is that they generally hit better than average catchers.
Again, I only had 11 observations. So the conclusions from this might be limited. Maybe this analysis will give other people some ideas on how to approach this issue.