Correct-a-mundo.... If you post a cover, buy the license so you are covered in a legal sense.
Doesn't matter how big the infringement is.... a few seconds in a sample or the entire song redone by you.... if you don't have permission... you are liable. And there is no such thing as "fair use" for musicians. You can post it and let it stream for free making no income and if and when they decide to come after you for infringement, they can literally recover THOUSANDS of dollars in past due royalties and license fees, and the judge will grant it.
It has happened in the past. Most often they simply send a cease and desist and tell you to take it down, but they don't have to. They can proceed directly to collection of royalties as THEY determine them to be. Not a fun day for you.
Harry Fox Agency can get the license you need to stay on the right side of the law, and as Beagle said, it doesn't cost much.