In his first two seasons with the Rams, Klein served as the backup tight end to veteran Billy Truax. While he appeared in all 14 games in his rookie season, he had only 2 pass receptions as he was primarily a blocker in the Rams' run oriented offense. However, he did catch a touchdown pass in the Rams 23-20 playoff loss in Minnesota to the Vikings. After the 1970 season, the Rams traded Truax to the Dallas Cowboys for wide receiver Lance Rentzel, and Klein took over the Rams' starting tight end position. Klein, an excellent blocker, also expanded his repertoire so that he averaged 21 receptions per season from 1971-76.
After the 1976 season, Klein was traded to the San Diego Chargers. Playing in the Chargers' high powered passing attack led by hall of fame quarterback Dan Fouts, Klein caught 91 passes for 8 touchdowns from 1977-79. Klein was then replaced by future hall of fame tight end Kellen Winslow and retired after the 1979 season.