Jesus never prayed for the sick, He commanded sickness to leave.
"....And when Jesus saw her, he called her to him, and said unto her, Woman, thou art loosed from thine infirmity." - Luke 13:12
"After leaving the synagogue that day, Jesus went to Simon’s home, where he found Simon’s mother-in-law very sick with a high fever. “Please heal her,” everyone begged. Standing at her bedside, he rebuked the fever, and it left her. And she got up at once and prepared a meal for them." - Luke 4:38-39
We don't have to persuade God to be merciful.
God expressed His mercy when He sent His son to die on a cross for us. His desire for us, His children, is wholeness.
Acts 17:11 "... because of his great love for us, God, who is rich in mercy."
God expects us to use the authority He has given us and command sickness to leave.