Monday's global sell-off, sparked by fears over a potential Greek exit from the euro zone and a Puerto Rican debt default, could be just the start of a big move lower for stocks, according to Dennis Gartman of "The Gartman Letter."

"Stocks are in real trouble here, and there's more trouble ahead," Gartman told "Fast Money" on Monday, after the Dow recorded its second-worst day in three years with a 350 point plunge. The move was accompanied by a sell-off in global sovereign debt.

Stocks rebounded slightly on Tuesday, but Gartman remained cautious about any short-term rally.

"We are very seriously concerned about the veracity of the global bull market," Gartman wrote in his Tuesday newsletter.

Markets were hit hard around the world on Monday as the drama in Greece reached a boiling point. Credit rating agency Standard & Poor's lowered its sovereign credit rating on Greece, and said the probability that the country exits the euro zone is now about 50 percent.

As for where investors can put money now, Gartman said to look towards the currency markets. After plunging off the initial reports in Greece, the euro reversed sharply, a move that Gartman says "caught a lot of people off guard."

By Gartman's reasoning, a so-called Grexit could actually give the euro a boost.

"I think the euro's going to get much stronger because they're going to toss the Greeks out," Gartman said. "Now is the time to buy the euro."

—By CNBC's Michael Newberg