^^ Yes, back when he was a current artist? One who is, by definition of no longer being with us, becoming less current all of the time? He achieved enough number ones in his life, and those were the actually important ones.
Having said that, there's also the point that the wider public who are the ones who actually dictate whether an album's a megaseller or not had already had their fill by MJ by the time 'Michael' came around, I think. The mad resurgence in his sales that occured after his death had only recently died down, This Is It had only been released on DVD earlier in the year. Plus, promotion was poor. Those who did know all that much about the album must've thought, "what? He's back again?", lol.
Plus, 'Michael' turned out to be an overhyped odds-and-sods album, really... Not the polished-off, Michael-approved, fully-fledged Invincible successor that we were really expecting. Hence why I don't consider the album a commercial failure, given both the circumstances and what it actually was.