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Lee: This may be one of 'few buying opportunities' left

The Dow and S&P 500 turned negative on the year as fears of a Greek exit slammed stocks around the world.

But despite the sharp selloff, one of Wall Street's most consistent and accurate bulls says the selloff has created one of the best opportunities to buy stocks all year.

"The Grexit is a source of volatility but not a thesis changer," Fundstrat founder Thomas Lee said Monday on CNBC's "Trading Nation." And while Lee acknowledged that U.S. investors could be in for a rocky few days or even weeks, he advised that "patience is needed but ultimately [this selloff] strengthens the case for a catch-up trade."

Read MoreStocks ignored Greece, now paying the price

According to Lee, Greece is not a large external threat to the U.S., as "only $22 billion or so of Greece's $240 billion in debt is held by either European or U.S. banks and the rest is ECB, IMF and euro zone governments." As far as Lee is concerned, Greece is contained.

"Greece will remain a Greek tragedy."

Lee added that "the world is better prepared" for a Grexit today than it was three years ago. "It's kind of unfortunate for the timing of this to happen because it was an adverse development, but I don't think this is going to change the trajectory of the U.S. recovery," said Lee. "It's not going to have an effect on the bull market."

Read More7 things you need to know about the Greek crisis

The bull insisted that investors shouldn't expect a European recession or "catastrophic" result of the crisis. "Fears of contagion will weigh, but lasting damage would require a Grexit to lead to a sufficient downturn in euro zone activity to drive a recession."

Lee, whose bullish target on the S&P 500 of 2,325 is among the highest in the Street, insisted that this may be of one the "few buying opportunities" left on the year. "Bottom line, we think any weakness is short lived and we are buyers into year end," said Lee.

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