Difo, 23, joins the Nationals for the third time this season. Most recently recalled on June 28 as the 26th-man for Washington's doubleheader at Philadelphia, Difo has appeared in six games at the Major League level in 2015. He collected his first Major League hit in his first MLB at-bat, May 19 vs. New York Yankees.
Difo entered 2015 rated by Baseball America as the No. 7 prospect in Washington's chain and was cited as the "Best Defensive Infielder" in the Nationals' Minor League system by the same publication. In 63 games between Single-A Potomac and Double-A Harrisburg this season, Difo is hitting .296 (74-for-250) with 19 doubles, one triple, three home runs, 32 RBI, 14 walks, 14 stolen bases and 34 runs scored.
Barrett, 27, returns to the Nationals after missing 24 games with a right bicep strain. The right-hander did not allow a run and surrendered just three hits in 3.0 IP over four rehab games with Single-A Potomac. Before his injury, Barrett was 3-2 with a 5.06 ERA in 30 games out of the Nationals' bullpen.
Since his Major League debut on Opening Day in 2014, Barrett's 11.18 strikeout-to-walk ratio is the 15th-best mark among National League relievers.
Span, 31, ranks 10th in the National League in batting average at .304 through 59 games this season. He's collected 15 doubles, five home runs, 22 RBI, 24 walks and 37 runs scored while going 11-for-11 in stolen base attempts.
Zimmermann, 29, is 7-5 with a 3.05 ERA this season. The two-time All-Star has been particularly strong of late, allowing just one earned run in his previous three starts, a 0.40 ERA (1 ER/22.2 IP). Zimmermann and his wife, Mandy, are expecting their second child.