US investors wary of Greece

"Greece never left thee EU in 2012, but the threat of it in June 2012 took U.S. equities down anywhere between 9 and 11%," commented McDonald, the head of U.S. strategy at Newedge. "We just went through that, and I think this is definitely a part of why we sold off, because the situation in Greece is definitely impacting the euro zone."

McDonald notes that the Greek concerns can be seen in Greek credit default spreads. As fears of a new Greek default have increased, those spreads have widened. In addition, he notes that spreads in France, Spain and Italy have widened alongside them.

"So what's happening in Greece clearly is taking other countries with it wider," McDonald said Tuesday on CNBC's "Futures Now". "That tells us that the legs and the length of it will depend on this credit situation normalizing."

Source: CNBC