New York began Wednesday with a 22-25 record, 5½ games behind the National League East-leading Philadelphia Phillies.
Mets manager Mickey Callaway said he was willing to go against the trend of limiting closers to three outs in most situations.
"We're doing this earlier than we would like, but you might see Diaz for four outs,'' Callaway said. "If we're not going to be where we want to be at this point, how can we expect to be there and hold off on doing that with Diaz? So, we got to win some games. We're going to approach this like it's the playoffs because we feel like we need to.''
Callaway announced the decision shortly before the Mets' outfield absorbed another loss, when Brandon Nimmo was placed on the 10-day injured list with a stiff neck.
Nimmo joins left fielder Michael Conforto -- on the seven-day injured list with a concussion -- and Yoenis Cespedes, who likely will not see the field again until next year after breaking his ankle in an accident on his ranch.
Nimmo, who has struggled at the plate all season, had an MRI on Wednesday after experiencing a sharp pain in his neck earlier in the week. He is hitting .200 with three home runs and 14 RBIs in 130 at-bats, far below last year's numbers.
The Mets called up journeyman Rajai Davis to take Nimmo's spot on the roster. Pinch hitting Wednesday, he hit a three-run homer in the bottom of the eighth to help the Mets beat the Washington Nationals 6-1.
Davis, 38, was hitting .277 with three homers, seven RBIs and 14 steals for Triple-A Syracuse.
Right-hander Seth Lugo is on the injured list because of right shoulder tendinitis, and Jeff McNeil was out of a starting lineup because of a tight left hamstring. The Mets also designated right-handed pitcher Paul Sewald for assignment.
Diaz, a 25-year-old right-hander, has a 1.93 ERA in his first season with the Mets. He has pitched 18⅔ innings over 20 appearances and is perfect in 11 save chances.
On April 15, Callaway decided not to bring in Diaz after Philadelphia loaded the bases against Jeurys Familia, and Robert Gsellman relieved, forcing home the tying run with a four-pitch walk to Jean Segura.
Callaway said then he did not think using Diaz for more than three outs was necessary, but his mind has changed.
"You might see Diaz for four outs moving forward for the unforeseeable future,'' Callaway said with linguistic flourish. "We're not going to throw him seven innings. They've got to put a limit on it at some point. But you know what? Hey, if we're a tie ballgame and he goes out and pitches the ninth and throws 10 pitches, we might send him out for a second inning. So, it's not a hard cap.''
After acquiring Diaz from the Seattle Mariners in early December along with second baseman Robinson Cano, the Mets re-signed former closer Familia to a $30 million, three-year contract to be Diaz's setup man. Familia has a 6.50 ERA in 18 appearances
"We also felt like, OK, the eighth inning is going to be handled,'' Callaway said. "It hasn't been handled. So we have to have to adjust.''
"The last couple of nights, we've been in a situation where we felt like, man, if we'd have had Diaz, it might have made this a little bit easier,'' Callaway added. "Where we are at and where we want to get to is probably the biggest reason. We're not where we want to be. We haven't played the way we're capable of playing. So, we need to adjust what we were thinking and what our plans were and be a better team.''
Callaway, the Cleveland Indians' pitching coach from 2013 to 2017, will reassess Diaz's workload as the season progresses.
"There's closers that have done this the whole season if they're smart about it and communicate the right way and are always honest about how they feel and we're not pushing them too much,'' Callaway said. "You can't do it twice a week, every day, for the rest of the season, but you can pick your spots and probably make it happen.''
The Associated Press contributed to this report.