I think it took a bit more than a "niggle" to get players of that era off the pitch permanently ! In 1965/66 there were 8 substitutions, in 1966/67 only 1 (Terry Bush as you noted earlier) and then under the tactical "regime" 16 in 1967/68.
A "useless" fact is that the all time total of substitutions for city is now just about 4600 !