One of the reasons Ramos is at Sevilla may also be because previously, Ramos was at this club and perhaps wanted to retire from this club. Indeed, Ramos was a very strong defender in his time. However, age cannot be fooled. no matter what your skills and abilities are, when you are 37 years old, your abilities and durability will not be as optimal as before.
Most of players want to retire at a club they debuts their career if possible. Ramos made a decision to return to Sevila at very last years in his career and refused offers from Saudi Arabia. He no longer hope to be call for Spanish team but still return to Sevilla and play there. I believe it is not because he wants to win Europa League with Sevilla but he cares more about where he plays for and retires. Sevilla is in his heart and his dream becomes true to return to where he started and very likely he will retire there too.
Maybe he can still contribute to Sevilla, but not optimally full time. So, however, Sevilla still needs another defender who is stronger and still young at least.
He can contribute for Sevilla like he did for PSG but the club has to use him cleverly as at his age, his body no longer allows him to play in all matches at severe intensity.