I don't see a Grexit coming any time soon. Northern Europeans are making a killing off of Greece by maintaining it as their economic protectorate. Even if Grexit does happen in the end, I'm sure that the U.S.A will basically shower Greece with money and aid to keep them on their side.
I can't see the U.S.A simply letting Greece go when everyone knows that the next day Russia will be installing military bases on Greek islands.