It was an emotional match for the eight-time Ballon d'Or winner, who supported Old Boys as a youth and played for them from 1995 to 2000 before joining Barcelona.
Messi was removed in the 60th minute, replaced by 22-year-old American Lawson Sunderland, but he delighted home fans ahead of next week's start of the 2024 MLS campaign and Inter coach Gerardo Martino for his quality performance.
"I spoke to him after the game and he was fine. We had talked about playing between 45 and 60 minutes," the Argentine coach said.
The 36-year-old forward, who sparked Argentina to the 2022 World Cup title, returned to Miami's starting 11 after fighting through a groin injury that caused him to sit out a friendly in Hong Kong earlier this month.
Messi came off the bench in a later Inter Miami match in Japan but was back among the starters for the MLS club's final pre-season match and in healthy form.
"I see Messi fully recovered," said Martino. "We take him little by little because the idea is that he arrives well-prepared for the (MLS season). From what I saw today on the field and the way he accelerated and arrived at the area, I really saw that."
After a scoreless first half, Inter Miami grabbed a 1-0 lead on Shanyder Borgelin's goal in the 64th minute.
The 22-year-old Haitian forward leapt high over a defender to nod home a perfect corner kick from Finnish midfielder Robert Taylor.
Newell's equalized on Argentine midfielder Franco Martin Diaz's goal in the 83rd minute, a blast into the bottom right corner from the heart of the box.
It was an important night for other Argentine talent on Inter, and Martino.
"For many of us who have a very strong bond with Newell's it was a special and nice game, a friendly to enjoy," Martino said.
Mauricio Larriera, Newell's coach, and the Old Boys felt much the same.
"Emotions had to be managed, something very important in these cases because ultimately it's a football match," Larriera said.
"In the case of Lionel Messi, all the boys were happy and with that sense of Argentine belonging, it was necessary to dose and manage not only the energy but also the emotional part."
'High expectations'
Inter Miami will play host to Real Salt Lake on Wednesday in the MLS regular-season opener after a global pre-season tour that saw the team struggle with injuries but ultimately build confidence for the upcoming season, Messi's first full campaign with the Florida squad.
"It was a challenge. From the football side it was not the best pre-season," Martino said. "Nor from the competitiveness side fundamentally because we did not have a complete squad to face the number of games we had... because of some injuries.
"We were aiming more to come out of the pre-season unscathed, to have the squad come back relatively healthy.
"We've met the needs the club had and we have worked to the start of the season. I have high expectations of arriving well at the beginning of the competition."
