What is the ruling on praying behind an imam who cannot perform rukoo' (bowing), or sit between the two prostrations, or for the tashahhud, due to illness or old age?

Bowing (Ruku') and sitting between the two prostrations or for the Tashahhud are pillars of prayer. If someone is unable to perform them perfectly due to old age or illness, then leading the prayer (imamate) by such a person is disliked (makrooh). It is preferable to remove this Imam and appoint someone else who can perform the pillars correctly, although the prayer behind him is valid. However, if he is unable to perform the very essence of these pillars, then, according to the preponderant opinion, prayer behind him is not valid.