While they go by Russia now, they're certainly acting like the Soviet Union. The crap Putin pulled with the Ukraine can be pulled anywhere, and the rest of the world needs to stay on guard. There is plenty of risk that Russia is back on a path to its roots of trying to strong arm the world by taking over whatever they can in the region to gain resources and power.
Yes, the United States should be worried about the possibility of the Soviet Union attacking Bulgaria because Russia formerly known as the Soviet Union has become more aggressive in occupying territory formerly controlled by the Soviet Union. Russia has been accused of helping incite ethic Russians in Ukraine, Estonia and has also invaded Ukrainian controlled Crimea in 2014.
I'm more concerted about the roman empire invading china. They are are still very powerful. We need to need a look out for those dead Romans. The soviet union arguably may also pose a threat but not nearly as much as a other dead empire. So Rome is our biggest prority
The United States should not be worried about a Soviet Attack of Bulgaria. While we do have interests in Eastern Europe, the country at a whole is at a crucial time and cannot afford to lose focus of itself first. The U.S. should definitely be proactive and attentive to any situation that occurs between two countries, but at this point, if it does not directly impact the United States, it should not be a worry.
This is a ridiculous question as the Soviet Union hasn't existed in any form since 1991. If it's just Russia, though, the United States would do well to mitigate Putin as much as possible. He seems to enjoy taking just for the sake of doing so, and though I don't imagine any massive war erupting, it's best not to let superpowers do as they please.