Carrasco, 36, was lifting weights before Sunday's game against Seattle when he set down a 50-pound dumbbell and it flipped over, landing on his pinkie, Showalter said before New York played at Washington.
He had surgery the same day to set the bone and a follow-up procedure on Tuesday to insert a pin.
"He's going to be fine to pitch again next year," Showalter said. "By the time he's ready to throw, he should be OK."
Carrasco went 3-8 with a 6.80 ERA in 20 starts this season. He recorded only five outs in his most recent appearance, a loss to the Los Angeles Angels on Aug. 26 when he was pulled after he allowed five runs on seven hits.
"He was wild in the zone a lot," Showalter said. "He had some good outings this year, but not nearly as many as he would have liked or we would have liked."
A year earlier, Carrasco went 15-7 with a 3.97 ERA in 29 starts for a Mets team that went 101-61, his best season since 2018 with Cleveland. He was diagnosed with leukemia the following May and returned to the mound three months later.
Carrasco is 107-93 with a 4.04 ERA in 14 years in the major leagues. He entered the season as part of a veteran rotation for a team that carried a record payroll of $365 million but fell dramatically short of expectations. New York traded its top two starters, Justin Verlander and Max Scherzer, at the deadline.
Carrasco will become a free agent after the season.
"He's going to pitch again next year for somebody," Showalter said. "We'll see whether it's for us or not."
Carrasco was placed on the injured list and right-hander Sam Coonrod was recalled from Triple-A Syracuse.
In other news, closer Edwin Díaz, who tore his right patellar tendon on March 15 while celebrating after the final out of Puerto Rico's win over the Dominican Republic at the World Baseball Classic, will likely travel with the team the rest of the season if he has no setbacks, Showalter said.
There is still no timetable for his return, but Díaz remains hopeful he will pitch this season.