After graduation he signed with the NFL's Pittsburgh Pirates (now Steelers), playing there during the 1938 season. He led the league in rushing his rookie season and became the game's highest-paid player.He took 1939 off to study at the University of Oxford as a Rhodes Scholar, but returned to play for the Detroit Lions from 1940–41.
In three NFL seasons, he played in 33 games. He led the league in rushing yards in 1938 and 1940, and he was one of the first "big money" NFL players, making $15,000 a year. His career was cut short when he entered the United States Navy during World War II; after the war, he elected to attend law school rather than returning to football. He was elected to the College Football Hall of Fame in 1954.
So the "one-season career" in '38 was referring to his upcoming Rhodes Scholarship year at Oxford.
I thought they meant this was his rookie, i.e. only, year in the league.