Bell will play for the U.S. Team, while Diaz -- the Pirates' No. 11 prospect -- will be behind the plate for the World Team on July 12.
Bell is batting .318 with a .390 on-base percentage for Double-A Altoona. The 28th-ranked prospect in baseball, Bell also has more walks (33) than strikeouts (30) through 66 games. Over four seasons in the Minors, Bell has hit .303/.366/.449.
The switch-hitting first baseman has hit only three home runs this year, but the Pirates aren't worried about his lack of home run power.
"Absolutely zero concern to us," Pirates general manager Neal Huntington said last month. "We want him to be a really good hitter, and we believe the power that's in there will show itself."
Diaz impressed the Pirates this spring with a capable bat and outstanding defense at catcher, and those traits helped him earn his first trip to the Futures Game.
The 24-year-old backstop has hit .281/.344/.393 with three home runs and 25 RBIs at Triple-A Indianapolis, where he's split time behind the plate with Tony Sanchez. The lack of everyday playing time isn't ideal, but it may prepare Diaz for his future in the big leagues.
"Both of those guys probably are going to break into the Major Leagues as a guy that shares time," Huntington said last month. "So it's not a bad thing for them to learn how to stay sharp and learn how to stay fresh and stay ready when their opportunity comes."
The Pirates will send another representative to Cincinnati, as Altoona manager Tom Prince will serve on the U.S. Team's coaching staff.
Major League Baseball, in conjunction with the Major League Baseball Scouting Bureau, MLB.com, Baseball America and the 30 Major League Clubs, selected the 25 players currently on each team.
While Bell was the Pirates' only representative last year, they sent Gregory Polanco and Dilson Herrera in 2013. Gerrit Cole and Jameson Taillon were selected to play in the 2012 Futures Game.
The 17th annual SiriusXM All-Star Futures Game takes place at 3 p.m. ET on July 12, and it can be seen live on MLB Network and MLB.com, with Matt Vasgersian, Dan Plesac, Heidi Watney and Jonathan Mayo on the call. In addition, SiriusXM Radio will broadcast play-by-play coverage of the event live on MLB Network Radio (XM channel 89; Sirius channel 209). MLB.com will also provide complete coverage before, during and after the game. Fans can stay updated by following @FuturesGame on Twitter and can send/receive tweets to/from the U.S. and World dugouts during the game by tagging tweets with the hashtags #USDugout and #WorldDugout.