Will My Rig Run NBA 2K15 Smoothly on Low settings?

depends on how you define smooth, most likely you will be playing below 30fps even at low settings and resolution. that will be a bummer since you're going to spend $60 on a game and play with dismal graphic settings.
 

How low do I have to tweak my game to make it become playable?
 


Lowest settings defined
maybe you will get a 20 fps average
 
the lowest possible settings, even the previous gen version would run bad on your rig.

actually skip everything i've said, it will not be a pleasant experience. you can upgrade the card to r7 250x and get better results without breaking the bank.
 

is 20fps considered playable?

 

Nope its considered laggy
but atleast not unplayable
 

actually it ran exceptionally well on medium.

 
uhm no, it doesn't run exceptionally well on medium at least to my liking. well we both have different views on what is playable and like what i've said spending $60 on a game and then playing it at less than 30fps with dismal visuals, and for a sports game, that pointless.
 


yep
if the rig does not satisfy the game's minimmum requirements
then its recommended to upgrade rig first
spending another 60$ for laggy experience is waste of money