You mean in 1939 or thereabouts?
If the Soviet Union invaded Poland alone (as in no Germany invading from the west) and Poland could put their full force against them, I'm sure they could do quite well in repelling them for a while. Soviet tactics of the time basically consisted of "send 10,000 men at that trench in a zerg rush, then 10,000 more if they all get cut down". Finland did, and their army was tiny compared to Poland's army, which was one of the largest in the world was it not?
So, in an alternate reality, Soviet Union invades Poland, Nazi Germany supplies them with anti-tank weapons to deal with the SU's massive amount of tanks, Poland fights SU to a stalemate with huge casualties on both sides, Soviets give up.
That's one way it could work.