OAKLAND, Calif. — Jason Bay found himself in unfamiliar territory, competing for a big league job, and then landed on the Seattle Mariners’ opening-day roster.

Despite all the injuries and struggles, the outfielder still believes he can contribute at this level.

The Seattle Mariners designated outfielder Casper Wells for assignment Sunday and kept Bay as the club’s fifth outfielder, finalizing their 25-man active roster ahead of Monday night’s opener against the Oakland Athletics.

“It was kind of the first time I’ve really had to make the team,” Bay said Sunday before the Mariners went through an informal workout. “I actually kind of liked it. It was different but I never really thought otherwise. I’ve been comfortable with myself and what I’ve done. From the day I signed here, I was on the team in my own mind, and I still had to go out and hear that, and I’m glad that I did.”

Seattle also selected the contract of right-hander D.J. Mitchell to the 40-man roster, then optioned him to Triple-A Tacoma. By adding Mitchell, it allowed them to make the move with Wells, who was out of options and couldn’t go to Tacoma. Seattle has 10 days to trade, release him or send him outright to the minors.

Wells hit .189 with two home runs and 14 RBIs in 16 spring training games. Bay, who agreed to a $1 million, one-year contract in December, batted .321 with two homers and six RBIs in 18 exhibition games.

“I think part of it was in the offseason working on a few things and getting off to a good start,” Bay said. “No matter what anybody tells you, that’s one of the most important things anywhere. It lets you relax and you’re not swimming upstream a lot of times.”

Manager Eric Wedge plans to use all of his position players regularly. Bay’s strong camp and veteran experience made him a valuable addition, Wedge said.

“He’s a guy who came in in great shape and moved around great all spring,” Wedge said. “A short, quick swing. I like the way he moved around the bases, I like the way he moved around the outfield and I like what he brings to the clubhouse. Just a lot of positives there. Casper’s a good ballplayer, too, but ultimately it came down to a tough decision and we felt we made the best decision for the club right now and moving forward.”

Bay is thrilled to begin anew after three injury-plagued seasons with the New York Mets.

He is ready to prove himself again in whatever his role becomes.

“I had chances to go elsewhere and I’d probably get more playing time but I wasn’t really doing this to boost value for next year,” he said. “I was doing it to have fun this year, in whatever capacity that might be, if I have 200 at-bats or 600 at-bats.”

After signing a $66 million, four-year deal before the 2010 season, the three-time All-Star hit .234 with 26 homers and 124 RBIs over the past three years, including a .165 average with eight homers and 20 RBIs in 2012. He sustained concussions and rib injuries and was limited to 288 total games with the Mets.

The sides agreed to terminate his contract. He was owed $16 million for this season and a $3 million buyout of a 2014 option, plus the final $2 million installment of his $8.5 million signing bonus that was payable by next June. The agreement allowed the Mets to spread out the payments.

The 34-year-old Bay will compete for playing time off the bench with veteran Raul Ibanez. Seattle’s starting outfield includes center fielder Franklin Gutierrez, left fielder Michael Morse and right fielder Michael Saunders.

“I knew coming in that I wasn’t going to play 162 games this year,” Bay said. “It’s a different part of my career, I understand that. That being said, injuries happen, things happen. I’m very comfortable where I stand. What happens happens. I’m actually quite happy with that. It’s a fun clubhouse and there are a lot of reasons to be excited. And rather than look at it as what you’re not doing, look at is as what I am doing.”