There is, of course, a good case to be made that Martin isn't the next Lorenzo Cain. But even with his disaster of Zunino-esque proportions, Martin offered a 1.1 WAR In an injury plagued season at a premium defensive position. That is setting a very fine floor.
This is a guy who as a 24 year old had a 1.033 OPS in 260 AAA at bats.
There is upside, but there is also an acceptable floor. Jerry wants defense in CF at Safeco. Good idea. So, he basically traded 1.1 from a relief pitcher who pitched full time to get it in CF in part time playing hurt. He had to throw in a decent prospect. That's a pretty good days work, I'd say.
Sure, there are backup plans. But nothing will come up in January that is going to look like it will outperform this one: a healthy Martin patrolling CF and batting ninth.