This is best answered by taking a quick look at what defines a project and what role the PM takes, as described below. As you review this information, ask yourself these few questions:
Is your project mission critical? Is it expected to take more than 3 months in length? Does the project involve multiple departments requiring coordination between them? If you answered yes to 2 or more of these questions, then you might want to consider using a project manager.
A Project (as defined by PMI)
A temporary endeavor undertaken to create a unique product or service.
Project Management
The application of knowledge, skills, tools and techniques to project activities to meet project requirements.
Project Manager
The project manager acts as an integrator to coordinate and manage the activities associated with a project. The project manager is the center of the PM knowledge triad of scope, cost, and time, and integrates these three areas of knowledge associated with a project.