The ultimate success of the Red Sox in 2015 is dependent upon the performance of the oldest player on the team. 

No, not David Ortiz, but rather closer Koji Uehara.

Uehara will turn 40-years-old on April 3, while Ortiz doesn’t turn 40 until Nov. 18. With the Red Sox’ rotation not being as strong as in years past, they will rely heavily on their bullpen, and no one more than their closer in Uehara.

After taking the baseball world by storm in 2013 on the way to the Red Sox winning the World Series, and carrying it over to 2014, Uehara hit a wall last August and September.

The closer had a miniscule 1.88 ERA over the first four months of the year, but that ballooned to 5.56 in August and 6.23 in September. His struggles were so bad he was taken out of the closer’s role and was shut down for a period of time.

Even with his rough final two months, the Red Sox made him their first priority of the offseason, re-signing him to a two-year, $18 million deal in late October.

“It was more physical,” Uehara told reporters last week of his struggles late last year. “I didn’t talk about it at that time. I think I’m over it ... I’m not going to go into specifics, but it wasn’t fatigue.”

Putting the last two months of 2013 in the past and proving they were just two outlier months are very important when it comes to the success of the 2015 team. Uehara doesn’t even have to be like he was in 2013 – when he recorded a historic scoreless innings streak of 30 1/3 innings – he just needs to serve as a dependable closer.

If Uehara does struggle, the Red Sox don’t really have a back-up plan for the closer’s role. Edward Mujica showed promise, but was inconsistent as a whole. Junichi Tazawa admitted during 2013 he doesn't feel comfortable closing games. The Sox signed reliever Alexi Ogando this past offseason, but he hasn’t thrown a pitch in a Boston uniform yet.

With how explosive the Red Sox’ offense is expected to be this year, they are sure to have many leads going into the ninth inning. Nothing is more demoralizing for a team than blowing leads in the final inning, and that is where Uehara comes in to play.

It’s pretty simple. If a team’s closer has a good season, more often than not the team has a good season. So as the 2015 season gets closer and closer to kicking off, the Red Sox are hoping this year Koji time also means winning time.