I used to have a very stiff lower back the next day after padel.
After some research, I now focus on strengthening my glutes and hip flexors in the gym. Hip thrusts ftw!
It has really helped me, I can now easily play on consecutive days without being stiff the next day.
(Keep in mind: Lower back pain has multiple potential causes. What worked for me does not necessarily help you)
my PT told me the same too, ive been doing glutes exercises too using bands