There are a few things to consider:
the outer discoloring isn't such a big problem
did is smell OK? is your fridge COLD?
ground beef or any meat that's broken done into smaller portions is more likely to become unsafe quicker
all in all ... if it didn't seem bad and you cook it at a very high temp you shouldn't become ill
but as a matter of preference always freeze if you aren't going to use within 24 hours of purchase
for frozen meat thaw by leaving in fridge covered for 12 - 24 hour