Generalized unitarity is a powerful method for doing perturbative calculations. We will discuss how to apply it to worldsheet S-matrices for the Green-Schwarz string in curved space. The studied theories are believed to be integrable to all orders in perturbation theory and symmetries have been used to determine exact S-matrices up to an overall dressing phase. Generalized unitarity can provide a simple way to check the proposed S-matrices perturbatively and to find dressing phases. As an example we will focus on $\eta$-deformed $AdS_5 \times S^5$