The rehabilitation process after knee injuries is often challenging for patients and requires a high level of resilience, as it involves the frequent repetition of mostly monotonous exercises. Based on recent research, serious games can significantly improve motivation by merging exercising with entertainment aspects and even combining it with hardware to apply external tasks and track the progress. The aim of this research is to propose and evaluate a new serious game pattern. The development is performed using systematic feedback from domain experts. The test setup involves analysis of patients’ feedback. The final game comprises an interaction with a balance board and an attached smartphone. Evaluation showed two main results. From a technical point of view: sensors of a standard smartphone (and it’s sensitivity) paired with a PC and its screen are usable in a rehabilitation setting. From a psychological point of view: the motivation to perform the knee rehabilitation process can be enhanced with a serious game delivering entertaining aspects to it.