NEW YORK ( TheStreet) -- The U.S. economy has struggled to grow since the end of 2007, when the the Great Recession began.Pimco founder and Co-Chief Investment Officer Bill Gross said in a letter to investors published Monday that, as a result, they will be hard-pressed to produce returns on stocks or bonds much above 5%. Gross, who manages the world's biggest bond fund at Newport Beach, Calif.-based Pimco, said investors "should avoid longer-dated issues where inflation premiums dominate performance" and consider "migrating to the relatively safe haven of 1- to 10-year maturities." In addition, agency mortgages are "back on the Fed's menu and may be a featured 'special' in months to come." What the world needs now, Gross says, is growth. An expanding global economy will help alleviate a debt hangover still plaguing the U.S. and, of course, Europe. The lack of growth, he says, is a structural, rather than a cyclical, problem. That means central banks' efforts to promote consumption by lowering interest rates and flooding the financial system with money are largely ineffective. Excessive debt compounds the issue, hobbling growth, whereby the need to pay down debt depresses a country's ability to use the tools needed to drive expansion in gross domestic product, a country's sum of all goods and services. Below are the five biggest worries Pimco's Bill Gross has to face in the search for growth, expanding upon his concerns outlined in his investor letter. 1. The ability of European and U.S. policy to drive growth With few details explaining how the Eurozone is going to execute its grand bailout plan, uncertainty will continue to rule the day. That's causing consumers and companies to cautiously plan their budgets for the next few quarters. Furthermore, accommodative monetary policy in the U.S. has set interest rates to zero and quantitative easing 1, 2 and a "twist" have been used, yet the U.S. economy is still only grinding along. If those policies aren't garnering the growth needed to jump-start economies, what will? 2. High unemployment The unemployment rates in the U.S. and U.K. have remained stubbornly high since increasing in 2008 with the start of the Great Recession. In September, the U.S. unemployment rate stood at 9.1%, below October 2009's peak. The U.K. is no better off, with a jobless rate at 8.1%, the highest in 17 years and heading higher. Not helping matters is companies outsourcing to emerging economies. Uncertainty about revenue is prompting companies to focus on widening profit margins -- hiring cheap labor overseas is one method. Without companies hiring locally, the unemployment rate will remain elevated, straining consumer spending.