Cotton decided to have the surgery after being diagnosed with a sprained ulnar collateral ligament in his throwing arm this week.
As seen in the following video courtesy of Martin Gallegos of the Mercury News, Cotton expressed disappointment at the injury:
Per Lee, Cotton had been optimistic about avoiding surgery since he felt improvement in his arm after leaving Monday's start against the San Francisco Giants.
Cotton discussed the situation further with Lee:
"It's tough, because this year I wanted to come out and prove to these guys that I can pitch in the big leagues, and I think I've done everything right to get better for this year. Now, it's a little setback right now.
"It wasn't one pitch. I don't know when I felt it. I didn't feel no pop, I didn't hear anything. I didn't have any numbing sensation. My elbow has full strength."
Cotton seemed likely to earn a spot in Oakland's starting rotation, as he posted a 3.75 ERA and 1.17 WHIP with 16 strikeouts in 12 innings over four outings in spring training.
The 26-year-old struggled last season for the A's to the tune of a 9-10 record with a 5.58 ERA, 1.44 WHIP and 105 strikeouts in 129.0 innings across 24 starts.
Oakland acquired Cotton in a 2016 trade with the Los Angeles Dodgers that sent starting pitcher Rich Hill and outfielder Josh Reddick to L.A.
In 2016, Cotton flashed massive potential with a 2-0 record, 2.15 ERA, 0.82 WHIP and 23 strikeouts in 29.1 innings over five starts for the Athletics.
With Cotton out, Daniel Mengden, Paul Blackburn, Andrew Triggs and Daniel Gossett are set to battle for the final three rotation spots behind Kendall Graveman and Sean Manaea.