Diaz, who played in Atlanta the last five years, returned to the Braves and was immediately inserted into Gonzalez's starting lineup for Wednesday night's game against Washington.
The Braves, who feature such left-handed hitters as Michael Bourn, Brian McCann, Freddie Freeman and Jason Heyward, need help from the right-handed hitting Diaz to help the team against tough left-handed pitchers.
Diaz hit .335 against left-handers with the Braves from 2006-10 and is hitting .295 against left-handers this season. He was put in right field on Wednesday night against Nationals left-hander John Lannan.
"What a big piece to have on our bench or off the bench or playing against certain left-handers," Gonzalez said. "It's something we haven't had, really."
Diaz entered the game hitting .400 (12 for 30) in his career against Lannan.
The popular Diaz was greeted with hugs when he entered the clubhouse on Wednesday, just in time for batting practice.
The NL wild card-leading Braves acquired Diaz for a player to be named and cash considerations. The deal was completed in time for 33-year-old Diaz to be eligible for the postseason roster.
Diaz said he suspected he might be traded when he wasn't used by the Pirates in his normal pinch-hitting role on Tuesday night.
"I couldn't sleep much last night, thinking about where it would be," Diaz said before adding he was told of the trade on Wednesday morning.
"I got the best news I could have imagined," he said. "Got on a plane within an hour and a half, I think."
Diaz was a part-time starting outfielder and one of the Braves' top players off the bench. He left after the 2010 season to sign a two-year, $4.25 million deal with the Pirates.
Diaz hit .259 (56-for-216) with 12 doubles and 19 RBIs in 100 games with Pittsburgh. The Pirates had little playing time available for Diaz after outfielders Alex Presley and Jose Tabata recently returned from injuries.
"This works out absolutely perfect," Diaz said.
"Hopefully, baseball-wise, getting back here where I feel comfortable, people know my funky swing, we can get something to click."
The Braves placed left-hander George Sherrill on the 15-day disabled list, retroactive to Aug. 27, with left elbow inflammation, clearing a roster spot for Diaz.
Atlanta has an 8½-game lead over St. Louis for the NL wild card.