Wagner will be in uniform Thursday night and wear No. 13. Shortstop Alex Gonzalez, who previously wore No. 13, will switch to No. 3.
The 38-year-old was acquired from the Mets on Tuesday in exchange for two players to be named later.
Wagner has appeared in two games for the Mets in 2009, throwing two scoreless innings while striking out four. In 45 relief outings with New York last season, Wagner went 0-1 with 27 saves in 34 chances to go with a 2.30 ERA before undergoing season-ending Tommy John surgery on his left elbow on Sept. 10.
In 24 starts for Boston this season, Penny went 7-8 with a 5.61 ERA with 89 strikeouts in 131 2/3 innings.
