People knew not to put new wine in old wineskins or a new cloth patch on an old shirt. Likewise, Jesus' new way of life is incompatible with old habits and beliefs!
The Parable of the Cloth and the Wineskins concludes a much longer narrative. The context and reveals deeper meanings and applications of the parable.
Who or what is the new man? Charles Whitaker explains that the new man is Jesus Christ Himself, living in us by His Spirit!
Sin causes disease, but the person who becomes sick does not necessarily commit the sin. Because God alone can forgive sin, God alone can heal.