The weak-hitting Mets placed outfielder Michael Cuddyer on the 15-day disabled list because of knee trouble.
Conforto, the 10th overall pick in the 2014 draft, batted seventh and played left field.
"We're hoping he's a good fit," manager Terry Collins said. "I told him this is the same game played by people who might be a little bit better but there's a reason you're here — you must be just as good."
Cuddyer hasn't played since Tuesday because of inflammation in his left knee. The move was made retroactive to Wednesday.
The team didn't have a clear timetable for his return.
"I can't project that," general manager Sandy Alderson said. "We think he needs to get a couple of weeks off his knee and let it calm down."
The 36-year-old Cuddyer is hitting .250 with eight home runs and 30 RBIs in 82 games. He won the 2013 NL batting title with Colorado, then signed a two-year, $21 million deal with the Mets last November.
The Mets selected Conforto's contract from Double-A Binghamton. He was drafted from Oregon State and began his pro career last summer at Class A.
"Last year around this time, I was just getting started in Brooklyn," Conforto said. "I'm at a loss for words, but I'm excited."
Conforto was batting .312 with five homers and 26 RBIs in 45 games at Binghamton after beginning the year at high-A St. Lucie.
Conforto started in the All-Star Futures Game this month. He went 2 for 2 and threw a runner out at the plate.