If it's truly a baby, a baby Virginia opossum alone is orphaned/abandoned. This is one of relatively few species where this is true. The babies ride on the mother's back until they're ready to be independent. If they fall off, the mother won't come back for them.
Well, I say baby, but it was the size of a large/jumbo rat. Likely old enough to be on his own but much smaller than I am used to seeing. The toughest thing is keeping our small dog from chasing it down. She thinks she was born to hunt!