It is indisputable that LeBron James is Miami's best player, and nearly indisputable that he is the world's best player. Dwyane Wade is likely the second-best player on the Heat, having won a championship by himself in 2006 and having played better than anyone else in the 2011 Finals.
But despite all that, Chris Bosh might be Miami's most important player. Yes, it is possible (on this unique team, at least) for a squad's third-best player to be its most essential.
We saw this in the 2012 Eastern Conference Finals when Chris Bosh's absence led to a gummed-up Heat offense and unleashed 2002 Kevin Garnett.
Dwyane Wade and LeBron James have a skill-set/role overlap. In the event of injury, one can do a credible imitation of the other. Since it's a high-usage role, the Heat can comfortably transition to giving Wade or James more shots if either goes down.
Miami has no replacement for Chris Bosh, though. He is its only good big man, its only player capable of guarding a center while also providing good offense on the other end. The offense he does provide opens up the floor for other Heat players.
Chris Bosh's most obvious skill is his silky long jumper. Yes, he's a long two specialist, but that ability can obscure his work around the rim. When Bosh gets a head start off a pick and roll, he can dive towards the hoop, taking advantage of the opposing team's hyper focus on LeBron James.
Teams should be wary of tracking Dwyane Wade too much, as he is also quite adept at dishing to Bosh. Defenses must give Wade some breathing room, or they will cede a dunk to Miami's longest player.
Had Joel Anthony received that pass, it would have resulted in a bobble, if not an outright turnover.
Chris Bosh provides his greatest offensive service by spacing the floor with his aforementioned shooting ability. He was no timelier with this skill than Game 7 of the Eastern Conference Finals.
Against the Celtics, Bosh hit corner three-pointers. While those points were crucial to victory, the threat of Bosh's shot had the added bonus of drawing Kevin Garnett out of the lane and allowing space for LeBron's drives. In semi-transition, the threat of Bosh's jumper forces Ray Allen to not pick up James as he streaks to the rim.
In pick and roll, Bosh will often move away from the hoop in a "pick and pop" shooting position. This allows for other players to get open space, depending on where the defensive help comes from. The Miami offense, in many ways, is predicated on Bosh drawing defenses farther from the rim. LeBron James and Dwyane Wade are devastating rim attackers, so this action is essential to keeping the NBA's best team churning out points.