The trade had been held up Monday due to a medical issue with one of the players involved in an earlier version of the deal, Mets general manager Sandy Alderson confirmed.
Bruce, 29, had spent his entire nine-year career with the Reds. He leads the National League in RBIs this season with 80 to go along with 25 homers and a .265 batting average. He becomes the first player traded during the season while leading the league in RBIs since the stat became official in 1920, according to Elias Sports Bureau.
Bruce, who will join the Mets on Tuesday, is a lifetime .249 hitter with 233 home runs and 718 RBIs. He will be slotted behind Cespedes in the Mets' lineup, although Cespedes is dealing with a right quadriceps issue and was out of the lineup for a second straight day on Monday.
"He's been a run producer," Alderson said of Bruce. "He's been in the middle of a lineup. From my standpoint, are we relying on him to produce runs? Yes, I think probably we are. But I think his presence in the middle of the lineup will change things. It wasn't clear to me how much longer Cespedes was going to get pitches to hit as long as we had the rest of the lineup around him as it was."
In the trade, the Reds get 22-year-old second baseman Dilson Herrera and 19-year-old left-hander Max Wotell. Herrera batted .211 in 103 plate appearances with the Mets in 2015 and was hitting .276 with 13 homers and 55 RBIs at Triple-A Las Vegas this season.
Wotell, drafted by the Mets in the third round of the 2015 draft, was 3-1 with a 4.25 ERA in six starts for Kingsport of the short-season Appalachian League.
Outfielder Brandon Nimmo had been in an earlier version of the trade. Cincinnati officials had concern about a partial tear of a tendon in Nimmo's left foot that was suffered in late January, but it is not clear if that medical issue or another prospect's medical info forced the reconfiguration of the deal.
Herrera had been viewed as the second baseman of the future for the Mets. His presence had been one justification for not making a multiyear offer to Daniel Murphy last offseason when he defected to the Washington Nationals.
Alderson said circumstances have changed, though, and the Mets are equipped to cover second base in 2017 even with Neil Walker's expected departure as a free agent and Herrera gone. The Mets now have Jose Reyes under control for next season at the major league minimum. Alderson also cited Las Vegas shortstop Gavin Cecchini as a potential second-base option next season.
"It's not that our estimation of Dilson has gone down," Alderson said. "It's that we realize we have some other options and seem to be well covered there. This was an opportunity to deal from a position, I think, of relative strength in the system to acquire some offense that we felt we needed."
Bruce's contract has a team option next season worth $13 million, with a $1 million buyout. That should provide the Mets comfort since it would reduce the blow if Cespedes opts out of his contract in November. Alderson suggested the acquisition of Bruce was not made with the potential loss of Cespedes this winter in mind.
"We were not looking for a rental player, and certainly not looking to give up the kind of talent we did for the next two months," Alderson said.
The Mets were not on Bruce's no-trade list, which included the Red Sox, Rays, Yankees, Blue Jays, Diamondbacks, Marlins, A's and Twins.
Why did the Mets need Bruce? For starters, they rank last in the majors in batting average with runners in scoring position at .206.
The Mets boast one of the best pitching staffs in baseball, led by Noah Syndergaard and Jacob deGrom, but they are only 13th in the National League in runs scored with 381. Last season, they were in a similar position and acquired Cespedes, who sparked them to the World Series.
The addition of Bruce does leave the Mets with a collection of players who are not ideally suited for center field. Collins hopes Cespedes can return to center field once his quadriceps improves. Otherwise, Michael Conforto and Curtis Granderson will need to be used there against right-handed pitching.
"It's not an absolute perfect fit for us," Alderson said. "You start with the need for offense and work from there." Bruce homered in five straight games in late July, a career best. One downside to him as a player has been his career-long penchant for following a hot streak with an incredibly cold one.
The Reds tweeted "thank you" to Bruce.
Thank you, Jay, for all you've done for the Reds and our community. We wish you, Hannah & Carter all the best in NY. pic.twitter.com/O8ThOT4oZU— Cincinnati Reds (@Reds) August 1, 2016
Cincinnati is into its second year of rebuilding, trading any high-priced veteran it can. The Reds dealt starters Johnny Cueto and Mike Leake, All-Star Home Run Derby winner Todd Frazier and closer Aroldis Chapman in the past year.
They tried to trade second baseman Brandon Phillips to the Nationals, but he used his option to block the deal. They also put together the framework of a three-team deal that would've sent Bruce to Toronto in the offseason, but that fell through. Bastardo signed a two-year, $12 million deal with the Mets in January but had struggled in relief this season. Niese is receiving $9 million this season and is likely to be bought out of team options for the following two seasons.
Although Niese mostly served as a starting pitcher during his original stint with the Mets, Collins plans to use him out of the bullpen. He will provide a rotation safety net, though, in case Zack Wheeler has a setback in the final stages of his rehab from Tommy John surgery. Wheeler's fastball sat at 90 to 93 mph during a simulated game on Monday. He is due to begin a rehab assignment on Saturday.