Ryan, 28, has been one of baseball's premier defensive shortstops the past two seasons for St. Louis and has also played some second and third base.
Mariners general manager Jack Zduriencik said Ryan would compete for a starting position, most likely at either second or shortstop.
"What I've said to Brendan and [manager] Eric [Wedge] as we've talked through this is what I'd like to do is create opportunities and create competition and let the pieces fall where they may," Zduriencik said.
Ryan gives the club some options heading into Wedge's first Spring Training with the Mariners. He could fill in at second base until the club is ready to promote rookie Dustin Ackley, or provide depth and competition at shortstop, where Jack Wilson has missed considerable time due to injuries the past two years. Utility players Josh Wilson and Matt Tuiasosopo are the other returners in the mix at those positions.
Zduriencik said he hasn't talked to Chone Figgins yet about possibly moving to third base, where he played with the Angels before signing with Seattle last year. The Mariners have a vacancy at third after trading Jose Lopez to Colorado and that seems the logical move, with Ryan taking over at second while Ackley gets further seasoning at Triple-A.
"We would like to have discussions with our players in the next 7-10 days to give them the lay of the land, and then some decisions could be made," Zduriencik said. "But it's a little premature as we speak."
The 6-foot-2, 195-pound Ryan is entering his first year of arbitration eligibility and thus would be under the team's control through the 2013 season.
Cleto, 21, was added to the Mariners' 40-man roster on Nov. 19 to protect him from the Rule 5 Draft. The right-hander went 4-9 with a 6.16 ERA in 23 games for Class A High Desert last season and was 2-1 with a 7.91 ERA in six starts for Peoria in the Arizona Fall League.
Cleto was acquired two years ago as part of the three-team trade that brought Franklin Gutierrez and Jason Vargas to Seattle.
"We gave up a very nice arm to acquire this player, but when you can fill a need with a premium-position guy who can play multiple positions, that's somebody you've got to get," Zduriencik said.
Ryan played a career-high 139 games with the Cardinals last year, hitting .223 with 19 doubles, three triples, two home runs and 36 RBIs. He led all Major League shortstops with 3.43 assists per game and an .874 zone rating.
He hit .292 with two home runs and 37 RBIs in 129 games in '09 and has a career batting average of .259 in 415 games over four seasons, with 312 games at shortstop, 59 at second base, 29 at third and four in the outfield.
Ryan said wrist surgery shortly before Spring Training last year contributed to his struggles at the plate. He missed about the first month of camp and said he rushed to get back in the lineup because the Cardinals valued his glove.
"I got as many at-bats as I could [after returning in late March], but I didn't quite feel right," Ryan said from his home in Los Angeles. "I took that into the season and it didn't work out so well. I got myself a little lost, lost a little confidence. I couldn't find my '09 swing."
Ryan said his wrist feels "wonderful" now and he's eager to see what the Mariners have in mind, saying he'll play wherever needed.
Ryan described himself as primarily a gap-to-gap hitter who likes to get on base and create things. He stole 25 bases over the past two seasons.
The Cardinals began shopping Ryan at last week's Winter Meetings after acquiring shortstop Ryan Theriot from the Dodgers. The Twins were among several teams interested.
Ryan played college ball from 2000-03 at Lewis-Clark State in Lewiston, Idaho, leading the Warriors to the 2002-03 NAIA World Series championship before being selected by the Cardinals in the seventh round of the 2003 First-Year Player Draft.
He's eager to return to the Pacific Northwest.
"You can't get better summers than in the Northwest," he said. "Those summers in St. Louis will crush you at times. That's some kind of humidity and heat. It'll be familiar stomping grounds. Playing at [the University of Washington] and traveling all over the Northwest was a great experience for me. We had Mariners games on every night at the house, watching Bret Boone and some of those great seasons they had. Hopefully we can do something similar to that."
Though Ryan replaces Cleto on the 40-man roster, the Mariners still need to move someone off the 40-man on Monday after announcing the signing of free agent Jack Cust on Friday. Another 40-man roster move will need to be made when free-agent catcher Miguel Olivo officially signs.
Olivo has agreed to terms, but won't be officially added until he returns from the Dominican Republic and passes a physical.
Zdurincik indicated the addition of Ryan checks off one of the remaining targets on his to-do list.
"One of the things [we did] when we went to the Winter Meetings [is] we talked about some of our needs," he said. "We felt we needed to fill DH, bring in a catcher and add an infielder. That does not mean we're done, but it does mean we've gone down [the] road where some of the things we felt were necessary were addressed."