The 23-year-old Argentine, who led 2021 MLS regular-season scorers with 19 goals and eight assists, met his compatriot Maxi Moralez's cross at the corner of the goal area and his volley bounced over Atalanta goalkeeper Brad Guzan into the side-netting at the far post. Moralez also set up the second goal when Guzan deflected his shot on to the crossbar for Callens to head home from close range.
"He's flying and he's done that for almost the entire season now," said NYCFC coach Ronny Deila of Castellanos. "He is dangerous inside the box and has confidence. We have a team that plays up in good situations. I always say that nobody shines alone and when the team is good everybody shines. That is what the team is doing right now.
"Look at the performance of Sean (Johnson) today, two or three important saves, Tayvon Gray coming in at 19 years old and playing like he's 10 years older than he is. Players like Alfredo (Morales) and James (Sands) in the center of the pitch having a really good game. This is all because they do their tasks, do their jobs, and support one another. We play as a team. Of course, Taty has special qualities and gets himself into situations that allow him to do big things."
NYCFC has reached the conference semifinal four previous times in the last five seasons without advancing.
Nov. 21 in New York City
NYCFC 2 Atlanta United 0. Goals: Castellanos 49 (Moralez), Callens 53.
NYCFC -- Johnson; Gray, Chanot, Callens, Amundsen; Morales (Jasson, 90+6), Sands; Medina (Rocha, 89), Moralez, Rodriguez (Magno, 85); Castellanos.
Atlanta -- Guzan; Bello, Franco, Robinson, Lennon; Sosa (Mulraney, 62), Rossetto; Moreno, Barco, Araujo (Damm, 62); Martinez.
Red card: Atlanta United -- Franco 90+3.
Referee: Chris Penso.