After a first half in which the Steelers only managed a pair of field goals, Tomlin introduced the 24-year-old quarterback, who ran in for two rushing touchdowns to hand Pittsburgh a 20-10 lead.
However, Pickett also threw three interceptions as the Jets hit back, and rookie Breece Hall's game-winning touchdown reduced the Steelers to 1-3 for the season.
Pickett's introduction came as something of a surprise after Tomlin appeared to rule out a change at quarterback following the Week 3 loss to the Cleveland Browns, and he refused to give away his future plans after Sunday's game.
"I'm not going to talk extended as we sit here," said Tomlin. "We did what we needed to do to put ourselves in a position to win this game. We'll do it again.
"But I like to just keep it where we are in terms of what transpired here today. We'll deal with next week, next week."
Asked why he brought Pickett in, Tomlin added: "We just thought we needed a spark.
"We didn't do much in the first half, not enough offensively and thought he could provide a spark for us.
"I thought he did some good things. I thought there was some energy there. We scored some touchdowns. But obviously we also turned the ball over.
"We're disappointed, but what transpired is not anything mystical. That's what we talked about as a collective in there.
"No disrespect to the Jets. They made plays and won the football game. But it's not about who we play.
"It's not about rabbits' foots and so forth. We've got to play better. We've got to put them in a better position. We've got to perform better. And I'm talking about the collection of players and coaches in there."
Pickett, meanwhile, fronted up for Pittsburgh's defeat as he echoed Tomlin's call for improvements, saying: "I'm just disappointed.
"[I] wanted to come in and get the win so that's really just the emotion, just disappointed we didn't come out with the outcome we worked so hard for. Definitely something we need to learn from and move on.
"The plays were there, we made plays, but the turnovers killed us. It's definitely on me and I have to improve."