A couple of weeks ago on a White Sox game, Ken Harrelson said something like "back in the late 70s, when you were facing the Yankees, Reggie Jackson wasn't the guy you had to worry about. Other teams worried about Munson and Piniella in the late innings."
So I looked at how each of them hit in innings 7-9 from 1977-79 using Baseball Reference. The table below summarizes the data.
If you were afraid someone would get a hit, Piniella was the problem. But if power was the concern, Jackson was still definitely the guy doing the damage. And his average was not much lower than Munson's.
Here is the AVG/SLG/ISO for each guy combined for the three years:
Notice that if you add up the isolated power of both Munson and Piniella it is still less than Jackson's, meaning that he had more extra bases per AB than those two combined.