Nouriel Roubini, the economist known as "Dr. Doom," says the US economy is left with only two outcomes as a result of the most aggressive monetary tightening by the Federal Reserve in decades: a hard landing or runaway inflation.

In a television interview on Monday, Roubini noted that the federal funds rate should be above 4% - 4.5% to 5% in my view - to really push inflation down to 2%. "If you don't do that, inflation expectations are going to get really crazy. But if it does, the outcome will be a hard landing. Either way, you have a hard landing or runaway inflation."

He believes the Fed is not "hawkish" enough right now. He argues that even if rates were raised to 3.8%, US inflation would still be above 8%, and that inflation would fall only gradually and slowly. "To me, the market is expecting a shift in monetary policy -- the Fed starting to cut rates next year, which sounds delusional."

Roubini believes US inflation may have peaked, but the question is at what pace it will fall. At the same time, he believes that geopolitical events may cause inflation to rise further, and the Russia-Ukraine conflict may push up commodity prices again.