Complex sentence: He entered into a contract with the NFL's Minnesota Vikings as an undrafted free agent and was the backup to Fran Tarkenton from 1963 to 1966 and competed for the starting position after Tarkenton was traded to the New York Giants in 1967.
Split into simpler sentences: He entered into a contract with the NFL's Minnesota Vikings as an undrafted free agent. He was the backup to Fran Tarkenton from 1963 to 1966 and competed for the starting position after Tarkenton was traded to the New York Giants in 1967.