Rupp had been battling an Achilles injury in the leadup to Chicago and said that it flared up near the end of the race. Last week, he decided to get surgery "to correct a condition called Haglund's Deformity, a bony bump on his heel that was causing his Achilles tendon to fray."
Here's more from Ken Goe's article for The Oregonian:
Rupp's problems began in earnest the next day, when his left foot was so swollen he had trouble walking. After consulting with his doctors, Rupp decided surgery was the best option.
During the surgery Dr. Amol Saxena detached part of Rupp's Achilles, reshaped the heel and reattached the tendon.
Saxena said Rupp's Achilles had partially torn in two places.
"He's pretty amazing," Saxena said. "I've treated almost 100 Olympians. He definitely has the highest pain threshold of anybody I've treated."
Had Rupp put off the surgery, Saxena said, the Achilles could have torn completely.
"It's one of those things where it's better to do it when you can plan it," Saxena said. "Sometimes it just tears, and you can't plan for that. The recovery is a lot longer when you completely rupture it.
Rupp really wants to win the Boston Marathon but he'll have to wait until 2021 -- when he'll be 34 -- to get another shot at it as he won't be running a marathon next spring and will have to do the Olympic Trials in 2020.