Neymar ruptured the anterior cruciate ligament (ACL) and meniscus in his left knee on international duty. He underwent surgery earlier this month in a Belo Horizonte hospital.
The forward is responding "very well" but the recovery will be long and he must focus on the process, Lasmar told a press conference at Brazil's training center in Teresopolis.
"He's shown, from the first moment that he's following what we're proposing. He knows that it's a process that he's going to need all of us, physiotherapist, fitness trainer, so that we can have Neymar (back) as soon as possible," said the doctor.
Lasmar, who carried out the surgery, said it was not yet possible to set a date for the Al Hilal player's return.
"Injuries are individual. In general, there are associated injuries that can be just as important, such as a meniscus injury. All this makes the procedure more complex, with a slower recovery period. That's why we don't talk about a timescale," he said.
Neymar joined the Saudi Pro League in August for a reported 90 million euros, but the former Barcelona and PSG forward only played five games because he was struggling with muscle injuries.
Brazil, who sit third in the South American World Cup qualifying standings with seven points, face fifth-placed Colombia, with six points, on Thursday. They host leaders Argentina, who have 12 points, the following Tuesday.
Lasmar also said goalkeeper Ederson had withdrawn from the squad due to an injury he sustained playing for Manchester City in their 4-4 draw at Chelsea in the Premier League on Sunday.
"He suffered trauma to his foot in the final minutes and was able to continue the match but the pain became intense. The foot became swollen, making it difficult to walk," Lasmar said.
Athletico Paranaense keeper Bento has been called up to replace him in the Brazil squad.
