Correct. Real Madrid players, Barcelona players, and Atletico players must feel tired as well. These teams have played in many competitions, but they never showed that they are tired. So, the players' fatigue isn't the right reason for a continuous decline in Sevilla performance. There should be another reason, it may be their strategy, player composition or formation, or disharmony between coaches and players, or any other else. It is unclear since there is no explanation from Sevilla management. However, Sevilla is still lucky if they finish in 4th place. They are eligible for UCL competition in the next season.
Sevilla played quite well, they were even able to kick Man United out of UCL. If they look downhill I think there is something lacking in the team so they cannot perform stable. This is why some clubs at the beginning of the season looked so promising, but in the end they couldn't survive. As a football team, many factors have to do with achievement. I don't think they've been able to do all that well, so they can't defend. I'm sure if all goes well then a football club will be able to stabilize in one season and the next.