Although Brazil coach Tite said Neymar still needs time to reach peak shape, he insisted he is not endangering the striker's health.
"Yes, he will be playing. There's no sacrifice here," Tite said through a translator Thursday. "We want to win but the coach is not going to play around with a player's health, the price would be too high to pay."
Tite was expected to use the same 11 players that were held to a 1-1 draw by Switzerland in the team's opening match, but right back Danilo got injured in the team's final training session on Thursday and will be replaced by Fagner.
Neymar limped out of a training session on Tuesday after appearing to hurt his right ankle. But on Wednesday, the Brazilian soccer confederation posted photos and a video of Neymar training with the rest of the squad and said he was fine.
"It's an evolutionary process. You need to speed up the steps," Tite said. "He's already speeded up that process."
The world's most expensive player, who joined Paris Saint-Germain for 222 million euros ($260 million) from Barcelona, only just made it to the World Cup after breaking a toe in his right foot on Feb. 25.
After recovering from surgery, Neymar played in two World Cup warm-up matches and scored in both to take his international tally to 55 goals.
But he lacked sharpness when Brazil opened its World Cup campaign with the draw against Switzerland on Sunday. Although Neymar was fouled 10 times during that match, he was still noticeably below his best.
On Thursday, Tite became angered after he was asked if he had spoken personally with Neymar to encourage him to be more unselfish.
"Absolutely not. The information you got is not true," Tite shouted, his voice rising steeply as he called it a lie. "That didn't happen."
He then said it would be pointless to restrict Neymar's way of playing.
"I'm not going to take his initiative away from him in the last third of the field, take away his genius," Tite said. "The last third of the field is where you have to go to create chances, and I'm not going to take that away from anyone."
Anything but victory could leave Brazil, one of the title favorites in Russia, in a difficult position to advance from the group stage.
Serbia leads Group E with three points after beating Costa Rica 1-0 and next plays Switzerland in Kaliningrad on Friday. Serbia and Brazil then meet on Wednesday in Moscow.
Brazil has no margin for error against Costa Rica, which won its group at the last World Cup and reached the quarterfinals.
"It is a decisive game, of course," said Tite, who sat next to defender Thiago Silva. "You can't run away from that."
Silva will take over as captain from left back Marcelo because Tite is rotating the job. Silva was Brazil's captain four years ago, when he was criticized for breaking down before a penalty shootout in the round of 16.
In Russia, he's urging his teammates to be brave.
"The team is fully aware of what we are going to be facing tomorrow," he said. "They didn't start well but they are ready to recover. The fight is on."