There's several to choose from, but it's not wrong to say that Danny Valencia has been the Mariners biggest disappointment through 11 games. Acquired from Oakland in exchange for Paul Blackburn in the off season, Valencia was expected to platoon with Daniel Vogelbach at first base. The plan changed though when Vogelbach struggled in the string, and was subsequently sent down to Triple-A, giving the duties exclusively to Valencia.
The veteran Valencia was expected to be tough on left handed pitching, but also be serviceable against right-handed pitchers as well. Through the first couple weeks, Valencia has been pretty abysmal however. He's slashing just .132/.195/.158 with no home runs and two runs batted in. As a result, Taylor Motter had started seeing more starts and had performed well, but was pushed into shortstop duties when Jean Segura wound up on the disabled list.
As the games continue and Valencia continues to struggle, Vogelbach's name has started to arise. The big lefty is off to a good start in Tacoma, hitting .280/.406/.360 with no home runs and four runs batted in across the first eight games for Tacoma. He spent a little time with the Mariners last year and was listed as their 10th-ranked prospect entering the year.
While Vogelbach may not be a season saver, he certainly looks like someone who could provide an upgrade over Valencia at the moment. It's early, and there's still a good chance that Valencia works out of his funk, but maybe giving him some days off and putting a little pressure on him would benefit both him and Vogelbach as the Mariners try to overcome a horrible start to the season.