Plants do metabolize diclofenac (the specific mechanism is explained in the article below). This indicates that it’s possible to test if plants could react to painkillers while being damaged.
Metabolism of diclofenac in plants – Hydroxylation is followed by glucose conjugation
Aditionally, I think this is also relevant: there’s absolutely no evidence that plants are not sentient.
(Answered with the info and suggestion provided by the researcher Octavio Muciño)