Coutinho has not featured for Liverpool competitively this term and handed in a transfer request in an attempt to force through a switch to Camp Nou, although Liverpool are understood to have rejected three bids for the playmaker – the latest coming in at £119 million.
Coutinho still joined up with Brazil's national team for the forthcoming World Cup qualifiers against Ecuador and Colombia on August 31 and September 5 respectively and trained separately from his team-mates on Monday.
Amid reports in Brazil that he will be ready to play, Michael Simoni – Fluminense's medical director – spoke to the midfielder and concluded his long-time friend was suffering with an "emotional issue manifesting itself as back pain".
"Coutinho's back problem? He is okay now. The back problem was a matter of stress, it was something emotional," Simoni told Esporte after meeting with Coutinho in Rio.
"The uncertainty about his future led him to feeling stressed. Everything is solved now.
"Coutinho and I have been friends for years. I always treat his medical cases. All he needed was someone to give him a pep talk."
Speaking at the presentation of club-record signing Ousmane Dembele on Monday, Barcelona technical secretary Robert Fernandez said the club hoped to make another signing this week, further stoking speculation over Coutinho.
"He's a bit anxious about the possibility of changing club," Simoni said in a separate interview with ESPN Brasil. "Of course, we talked about this possible transfer, about the things that are in his heart right now.
"His back pains, in my opinion, are explained by this anxiety. Muscular tension normally does that.
"He can show up to train normally. I didn't do a clinical exam, he came to chat, he came to talk to a friend."