Harvey is expected to miss several weeks with a stress injury to his right scapula.
Lagares is expected to be out for an extended period as well after fracturing the IP joint in his left thumb on Thursday night on a diving attempt in center field.
Reynolds is returning to the Mets after going 4 for 23 with nine strikeouts earlier this season.
Nimmo will be on the major league roster for the first time in 2017. He hit .274/.338/.329 with 12 runs scored, one home run and six RBI in 32 big league games last season. Nimmo missed most of April this year with a hamstring injury he suffered while playing for Team Italy in the WBC.
Reynolds is slashing .286/.375/.393 with one home run and Nimmo is batting .223/.361/.378 with three home runs and 16 RBI for the 51s this year.