Dennis Schroder bet on himself ... and lost MILLIONS.
The free-agent guard just agreed to a 1-year, $5.9 million deal with the Celtics ... which sounds just fine -- that is, except when you take into account he REJECTED a 4-year, $84 MILLION extension from the Lakers just months ago!!!
BTW ... ESPN's Brian Windhorst said back in February the Lakers offered him the maximum amount they could ... but DS felt he could rake in more dough, which definitely didn't happen.
The 27-year-old announced the move via his Instagram page minutes ago .