The Federal Reserve's decision to eventually sell of the massive amount of assets it has bought to keep the economy afloat will be key to the future direction of the stock market, said Mohamed El-Erian, CEO and co-CIO of PIMCO

El-Erian doesn't think the Fed's winding down from $85 billion a month in bond purchases, aggressive forward guidance, and near zero interest rates will "come for quite a while." But when it does happen, he said, "It's going to be incredibly complex."