Ideally, the Mariners will at some point end up with a starting rotation of Felix Hernandez, Jason Vargas, Doug Fister, Erik Bedard, and Michael Pineda. That's the group we all want to see cycling through come May or June. But because of the uncertainty with Pineda and whether he's worth bringing up immediately out of camp, there's currently a competition for the fifth spot. And that competition no longer includes Nate Robertson.
Robertson's set to undergo elbow surgery. It's nothing severe like a torn ligament or anything, but he's having some loose bodies removed, and that'll keep him from throwing for about a month. From there, he'll have to build his arm strength back up, so Robertson isn't going to get into a game at any level until sometime in May at the earliest.
What that means is that the competition is down to Pineda, David Pauley, and Luke French. Even with his issues, Pineda is almost certainly the best of the three right now, but if Pauley or French is able to show that the gap isn't too wide, then he should be able to win a few regular season starts while Pineda smooths out some kinks in Tacoma. Nothing permanent, barring some predictable or unpredictable disaster, but enough to make some impression on somebody.
For Robertson, it's a tough break, since it seemed like he had a real chance, being the seasoned veteran and all. This is just further proof that you never want anyone to find out that you're hiding any bodies anywhere. Once those things get free it's nothing but trouble.