Alvarez, a 20-year-old catcher, was ranked as the No. 4 overall prospect in ESPN's midseason list published in August. He would be eligible for postseason action since the Mets have multiple players on the injured list.
Another Mets player joined the IL on Friday, as utility player Darin Ruf was placed on the list, retroactive to Tuesday, with a neck strain. Left-hander Alex Claudio was also designated for assignment.
Signed as an international free agent out of Venezuela in 2018, Alvarez has had a meteoric rise through the Mets' minor league system over the past two years. He slashed .260/.374/.511 with 27 home runs this season while splitting time between Double-A Binghamton and Triple-A Syracuse.
While he missed three weeks in August with a right ankle injury, he finished the minor league season by hitting .371 with a 1.107 OPS and three home runs in his final 35 at-bats for Syracuse.
A right-handed hitter, Alvarez could help the Mets at DH, especially against left-handers, against whom he hit .315 with a 1.019 OPS this season.
New York has received little production from Ruf and rookie Mark Vientos at designated hitter since sending J.D. Davis to the San Francisco Giants at the Aug. 2 trade deadline. Left-handed hitter Daniel Vogelbach is New York's regular DH against right-handers.
The Mets enter Friday night's series opener in Atlanta clinging to a one-game lead over the Braves in the National League East. The division winner earns a first-round bye, while the second-place finisher will be the top NL wild card and host a best-of-three postseason series starting Oct. 7.
Information from The Associated Press was used in this report.