Trader Brian Duffy reacts in the S&P 500 pit at the Chicago Mercantile Exchange on October 8, 2008.REUTERS/John GressMorgan Stanley CIO Mike Wilson says to consider rebalancing your portfolio.Concentration risks are growing as top stocks drive the market's performance.Investing in sector ETFs or other indexes are a couple ways to rebalance, Wilson said.If your stock exposure mainly consists of an S&P 500 index fund — as experts like Warren Buffett recommend for retail investors — it might be tempting to leave your portfolio how it is.