The Tenerife-born defender becomes the fifth player to leave Real Sociedad this summer, following Aritz Elustondo, Wesley, Caleta-Car and Brais Méndez out of the San Sebastián club, with De Telegraaf first revealing the move before it is expected to be finalised over the weekend.
All five exits reflect a squad that sporting director Erik Bretos has been actively trimming to fund incoming business.
Real Sociedad signed the 24-year-old from Deportivo Alavés for around €6.5m two years ago, viewing him as the long-term answer at left-back and tying him to a contract running until 2030.
He never established himself at Zubieta, however, enduring a difficult debut season before spending last year on loan at Real Oviedo, who were relegated from La Liga despite his efforts. With competition for his position still stacked on his return, new head coach Pellegrino Matarazzo used pre-season to assess him before ultimately giving the move to Rotterdam his blessing.
Feyenoord's need is immediate. The Dutch club have been left short at left-back with Jordan Bos still months from returning from injury and Gijs Smal only recently back in training, a crisis that saw 19-year-old academy product Thijme Wessels drafted into the squad for their Eredivisie opener at Sparta Rotterdam.
López is expected to provide senior cover and competition in that position from the moment the move is finalised, giving Feyenoord a proven La Liga-level option while their own left-back options continue their recoveries.
The loan gives sporting director Erik Bretos more room to complete the arrivals Real Sociedad still need before the window closes. The club are also working to bring in centre-back Nayef Aguerd on loan from Marseille, in a deal reported to include a buy option worth around €17m, as they continue reshaping the squad around returning boss Matarazzo, who guided the club to Copa del Rey glory last season and is now overseeing a significant summer turnover as he looks to build a squad better suited to his own ideas.