No player is irreplaceable. You can't afford to consider any individual player irreplaceable because no one is immune to injury (first-round picks Jacoby Ellsbury and Matt Kemp) or extended slumps (Tim Lincecum and Justin Upton).
However, these 10 players are either great in so many different areas that they might carry your team, or they are elite in a few areas to the point that they can almost win you a category by themselves. Losing these players is not a death sentence to the team (like losing an elite QB or RB can be in fantasy football), but will significantly hamper your title hopes.
Note: Don't look for closers here. There will always be more closers available.