According to the COBIT 2019 Design Guide:
"When outsourcing is selected as the sourcing model, managing relationships with external vendors becomes a top governance and management priority to ensure service quality, compliance, and accountability." This makesAPO08 Managed Relationshipsthe essential management objective for ensuring outsourcing success. While security and performance are important, managing relationships is the core requirement in an outsourced model.
Reference:COBIT 2019 Design Guide, Section 4.4.7