If your teammate who never misses practice starts missing, that’s not laziness until you’ve asked.

A change in somebody reliable is information about their circumstances rather than their character. The easy read is that they stopped caring, because that requires nothing from you. The likelier read is that something at home or in their head has got heavier. Judging first means you lose the chance to help and they lose a person they could have told.

Ask before you conclude, away from the group, and keep it plain: you have not been around, is everything all right. Do not push if the answer is thin. Say you noticed and that you are glad they are back, and tell an adult if it keeps going.