McDermott was a terrific pitcher in his early career (and Koufax wasn’t all that good at first). He burned out quickly, though.
I’ll add Otto Graham and Marion Motley of the Cleveland Browns. Both only played ten years, and both were dominating presences in their time.
Both of them got a relatively late start to their respective pro careers due to being in service during World War 2 before that, of course, similar to Jackie Robinson in that respect.
As noted above, ten years really isn’t an exceptionally short career for an NFL player.
On Koufax’s age comparisons: it is widely acknowledged that Sandy’s career was…weird. As a bonus baby, they couldn’t farm him out, so he missed out on the great training in Montreal and St Paul that characterized so many Dodger standouts of the era – he had to work out his early wildness on a major league bench, and he was only a marginal player for six years. He had a blazing fastball, but got behind on hitters and couldn’t stay in the rotation. Then he had a legendary eureka moment during spring training in 1961, adopting an easy motion instead of bearing down. Suddenly, he could hit the corners with that amazing lefthanded heat, and he was a different player for six years before putting down his glove due to chronic arthritis. His top ten comparable pitchers at age 30 are Roger Clemens, Jim Palmer, Tom Seaver, Lefty Gomez, Dwight Gooden, Dave McNally, Greg Maddux, C.C. Sabathia, Vida Blue, and Clayton Kershaw. Five Hall of Famers, the best pitcher of the last ten years, and two guys who were on the express train to Cooperstown until they pulled into Cocaine Depot, plus two guys who got off to much better starts than Koufax (Sabathia was 17-5 as a 20-year-old rookie), but didn’t do a lot past 30.
So yeah, by age 24 Koufax looked like a scrub who’d been moping around the league for six years. Then things changed.
I’ve mentioned Jack Iverson before in another context, but I think he fits here. Bear in mind that most of the other ‘short career’ athletes mentioned here started playing competetively as kids and worked their way up through local leagues - probably playing organised sport for 8-10 years before making the big leagues.
Jack Iverson was born in 1915. He played cricket as a boy at school (as everybody did) with no success (like nearly everybody) and then became a farm hand, eventually becoming a property manager. He enlisted in the war, and then while mucking about with his mates in the New Guinea jungle, he developed a fun, unique way of bowling. After the war, he thought nothing of it, until someone spotted him helping out with some blind cricketers. He joined the local cricket club in 1947, made it to the Victorian team in 1949, and represented Australia against England in 1950-51 (the peak cricket match-up in the world at that time) - age 35 - and was the key player in the series.
It would be equivalent to some nobody not involved in baseball at any level, developing a ‘mystery’ pitch at age 32 and joining a local, amateur weekends-only ballclub, attracting attention and getting to the major leagues at 34 and winning the World Series MVP and Cy Young at 35.
And then he got worked out - he had no real ability other than his ‘mystery’ ball - couldn’t bat, couldn’t field - and basically disappeared. He played a total of 2 top-level games (across 2 seasons) after his triumph.