Underground excavations in complex flysch formations are frequently affected by hydrogeological hazards resulting from lithological heterogeneity, mechanical anisotropy, and groundwater-induced degradation of weak rock masses. This study proposes a hybrid methodological framework that integrates the geotechnical mission requirements of EN 1997 (Eurocode 7) with Multi-Criteria Decision Analysis (MCDA) to support real-time prediction and selection of stabilization measures during tunnel excavation. The framework was validated through the analysis of a tunnel section excavated in highly degraded Category V flysch, where repeated hydrogeological failures provided the basis for evaluating the proposed approach. By combining geotechnical supervision, monitoring data, and MCDA-based decision support, the methodology enabled rapid identification and prioritization of the most effective mitigation measures, particularly advance drainage and systematic self-drilling anchor installation. The implemented measures significantly improved excavation stability, reduced ground deformation to acceptable levels, and prevented further collapse events along the remaining tunnel alignment. The proposed EN 1997-MCDA framework offers a practical and transferable decision-support methodology for managing hydrogeological risks in underground excavations, improving construction safety, reducing uncertainty in engineering decisions, and supporting more reliable tunnel design and construction in complex geological conditions.