Koji Uehara has 2014 vesting option

Boston Red Sox closer Koji Uehara has a previously undisclosed $4.25 million vesting option in his contract for 2014 that will be exercised if the 38-year-old makes one more appearance this season, a league source told ESPNBoston.com.

The Red Sox announced Uehara's contract last December as a one-year, $4.25 million deal with neither a player nor a team option. As it turns out, there was a vesting option in which an additional year would be guaranteed if he made 55 appearances in 2013. Uehara has appeared in 54 games this season.

The source indicated this kind of option was a highly unusual practice for the Red Sox.

Uehara, who has 12 saves after inheriting the closer's role following season-ending injuries to Joel Hanrahan and Andrew Bailey, has been the most reliable arm in Boston's bullpen this season. He has a 1.35 ERA with 74 strikeouts and just nine walks in 53 1/3 innings this season and has not surrendered an earned run in his past 18 appearances.

With Uehara all but guaranteed to be under contract for 2014, he could conceivably keep his role as the team's closer as long as he remains effective for the rest of 2013. Both Bailey and Hanrahan are free agents at the end of the season.

WEEI.com earlier reported the existence of the vesting option.