I think a crucial factor (working in our favor, luckily) is how Kobe will look at his teammates following this loss. IF he decides he wants to continue to try and lead the team, Lakers remain dangerous. IF he decides he should have gone through with the trade before the season, sulks, and does nothing to help his teammates coalesce into a team, it should be over on Sunday, with some funny theatrics on the Lakers side.
Frankly, whenever I read or heard Kobe say things like "WE have fun together", "WE love playing together", "WE are a bunch of young guys just doing our thing out there" etc., the "WE" always sounded hollow to me. Kobe seems neither a leader, nor just one of a group of teammates. Phenomenal player, to be sure, but not one to create or maintain team chemistry, which is something that the Celtics have.
If the situation were 2-2, it'd be far more dangerous. The Lakers could take comfort in evening the series, winning the last two matches etc., and would not need Kobe to bring them together. In a 3-1 situation, I predict (and hope) that Kobe will not rise to the challenge of getting his team together. He'll sulk and pout, but not re-create his team.
It's the Celtics to lose. I didn't know about Posey yelling at them that they still have not accomplished anything - that is great news that he did that. If the Celtics go out there, as a team, focused, playing like they belong holding that trophy this season, and just execute the game, I can't see Kobe inspiring the Lakers enough to defeat us. If the Celtics go out there all jittery, it'll be tough, but I still think the Kobe factor (with respect to his teammates, not individual skill) will help us.