Nate Robertson to undergo elbow surgery

The Mariners starting pitching race just was reduced by one with news that veteran lefty Nate Robertson will undergo arthroscopic surgery on his elbow Wednesday and be out for at least four weeks.

Robertson looked good in his first outing this spring, then struggled his past two appearances. Turns out he had some loose particles in his elbow that need to be cleaned out.

As a non-roster invitee, that’s a tough blow for the former Detroit Tigers starter. He obviously won’t be in the mix for the Opening Day roster and his future remains to be seen.

For the present, that leaves rookie Michael Pineda and veteran returners Luke French and David Pauley as the primary candidates for the fifth spot in the rotation. This assuming Erik Bedard stays on track as the No. 4 man behind Felix Hernandez, Jason Vargas and Doug Fister.

French and Pauley have both been good this spring, but it’s going to be hard to keep Pineda out of that spot unless the Mariners just really want to start him in Tacoma this year.