Greenwood was plagued by injuries in 1946, and in 1947 he suffered a debilitating cheekbone fracture that ended his professional career.

After retiring, he worked as a high school coach in Ohio and briefly as an assistant at Yale University.

In 1951, became the head football coach at Toledo University, but resigned from that position before completing his first season, saying the school had not done enough to counteract unnecessary violence in the game.

[5] Greenwood entered the United States Army Air Corps in 1942 to serve in World War II, but he got a medical discharge in 1943 after he was injured in a plane crash.

[10] He played in an October 1944 game against a military team at Great Lakes Naval Training Station coached by Paul Brown that ended in a 26–26 tie.

[12] Greenwood went undrafted in 1944 but signed late that year with the National Football League's Cleveland Rams.

[7] He came to Cleveland before the 1945 season and began an off-season job at Thompson Products Company in its engineering department.

The team won the 1945 NFL Championship Game by a score of 15–14 in frigid conditions at Cleveland Municipal Stadium.

But he left that post the same year in protest of "rough play" during a game between Toledo and Bowling Green.

[21] The cheekbone injury he sustained in an "uncalled for collision" in a 1947 game against the San Francisco 49ers helped prompt his decision, he said.