Jose Nunez on 1987-07-04 came in the 2nd inning and pitched the rest of the game 6 2/3 IP K'ing 9 guys. Thought he'd become something great at the time.
Most impressive: Steve Delabar in 2 IP K'd 6 (0 H 0 BB, 26 pitches 19 strikes - but faced 7 as someone reached base on a 3rd strike passed ball, extra out was grounder 1B-P)
For those curious and wanting to dig in (please post stuff here for fun) go to Baseball Reference.