Not a huge fan of Wally Pipp or anything, but I just don't get the comparison.
Pipp reportedly wanted a day off because of a headache. Manning has had all sorts of injury issues over the past year or so. Also, Gehrig, a future HOFer, took over for a pretty decent player. This is the reverse. Meanwhile, Pipp ended up playing 3 more years at a pretty high level somewhere else, while Manning is pretty much done. You could make more of an argument for Brady/Bledsoe. While Drew's injury was legit, Brady is a 1st ballot HOFer while Drew was at a pretty high level, and Bledsoe ended up playing out his career somewhere else for a bit longer. Still, not a great analogy, but moreso than the Manning/Brock one. Note: I'm not learning to spell Brock's last name until he plays well enough that he forces me to. It took me at least 4 seasons before I learned Roethlisberger.
Every injury causing an opening for a player is not a Wally Pipp scenario.