What Geneen meant by this is that the art of the manager is coming to terms with whatever situations develop in the course of running a business and choosing the course of action that makes the best of each. The world of the manager is one of problems and opportunities.
Opportunities are merely problems that promise to pay off after sufficient work.
Managers are not schooled in radical change. The elimination of entire industries and their replacement with others, the obsolescence of established products in periods measured in months, the consequences of continued exponential growth in technology are all foreign to the manager. Presented with a problem, an expert manager can quickly grasp its essence and begin to formulate a plan to manage the problem on an ongoing basis. This is not the domain of the manager.