The Seattle Mariners finished among the top half of American League teams in rotation ERA this year and are hopeful that they need to acquire only one starting pitcher before Opening Day, major league sources say.

That’s important for a team that must upgrade an offense that scored the fewest runs in the majors in 2010 and 2011. (And it’s not easy for an American League team to finish 30 out of 30.)

Seattle has Felix Hernandez and Michael Pineda for the top two rotation spots. Left-hander Jason Vargas is likely to return through salary arbitration, despite his 5.42 ERA in the second half. The question, then, is who the other two starters will be.

Blake Beavan showed this year that he can be a suitable No. 5 starter, going 5-6 with a 4.27 ERA in 15 starts. The last spot in the rotation could be his to lose, particularly since the front office will want some value from the Cliff Lee trade on their major-league roster. (The primary piece Seattle obtained in that deal, first baseman Justin Smoak, has been a disappointment.)

The Mariners have monitored Jamie Moyer’s offseason workouts, and the soon-to-be 49-year-old free agent may actually be a perfect fit for what they need. The team is excited about the possibility of one or more prospects — James Paxton, Danny Hultzen, and/or Forrest Snow — being ready for the rotation at some point in 2012. Moyer could stabilize the rotation until then.

Paxton and Hultzen are well-known prospects, but the tall, lanky Snow has been something of a revelation. Snow, a 36th-round pick in 2010, began this year in the low Class A Midwest League but advanced all the way to Class AAA Tacoma. He has since impressed scouts in the Arizona Fall League, where he hasn’t allowed an earned run in 11-2/3 innings.

— Jon Paul Morosi