Coutinho arrived as a free agent after six months without a club, having terminated his contract with Vasco da Gama to prioritise his mental health, according to Sport.
It was Neymar who picked up the phone and convinced his old friend to join him at Peixe, with Coutinho signing a deal running until December, when the South American season concludes.
Almost 20 years ago, Neymar and Coutinho posed for a photo together in the Brazil Under-15 dressing room, the first time their paths had crossed. This week at Santos, in the twilight of their careers, the pair recreated that photograph, Coutinho smiling again after a boyhood homecoming that ended abruptly and painfully.
He recalled the circumstances, still visibly affected, at his Santos unveiling.
"The year had started, we weren't doing so well, in the last few matches... the whole stadium insulting me, asking for me to leave. I know that happens, but it was very hard for me. Vasco was always my home. I made the decision to prioritise my health. I knew the consequences.
"I was completely honest with myself and with my feelings. I never abandoned Vasco. I was at my limit and couldn't have contributed anything like that," he said.
Coutinho was not alone in facing the media, with teammate Rodinei by his side for support, while the club scheduled the former Barcelona and Liverpool midfielder's appearance a few minutes before Neymar and Arthur arrived, in an attempt to reduce the media scrum.
He lamented the criticism that followed his departure from Vasco's supporters.
"The reaction was huge, and it makes me sad. Vasco, for me, was always my home. I was raised there, I played all the way through the academy, I came up to the first team. I always made clear my affection, my gratitude and my love for the club."
Coutinho stressed that sentiment, not money, drove his choice to join Vasco in the first place.
"When the chance to come to Vasco came up, I had another year left in Qatar and was earning $10 million clean."
He added that he had also given up a year remaining on his Aston Villa contract to make the move.
"I didn't want Vasco to spend a cent on a transfer for me. That was my priority. The only way was to give all that up - my salary for a year, and the money from Aston Villa. I left it all on the table and came. I had the chance to play in the Copa do Brasil final, we unfortunately lost, it was very tough. I'd won plenty of trophies abroad and came with the mindset of winning and becoming an idol. It wasn't possible."
Neymar publicly offered his support with a message for his friend: "I'm here to protect you and hug you, and so you can be happy again."
Forward Rony also made a gesture of his own, handing Coutinho the number 11 shirt he had originally been set to wear as number 19; Rony will instead take the number 33.