James did not travel with the squad ahead of Saturday's match, with Tuchel revealing that the defender had started an "accelerated rehab programme" as England looked to secure their place in the knockout stages.
"He has a minor hamstring injury," Tuchel said. "He was not able to train the last two days.
"He's on an accelerated rehab programme. We will take it game by game, but we strongly believe that he will be available for us again in the tournament."
England enter their final group match top of Group L on four points, level with Ghana, while Croatia were third on three points and Panama remained bottom without a point.
Tuchel admitted James' injury was unexpected after the Chelsea defender had been in good condition before the problem occurred.
"No-one could see that coming. Reece felt good and was in good shape," he said. "We would love to have Reece, he's a good player, and we will look for solutions. That's what we do."
The England boss added that James could be available for the round of 32, although he accepted the recovery schedule was tight.
There were also concerns over Bukayo Saka, Declan Rice and Elliot Anderson after the goalless draw with Ghana, but Tuchel confirmed the trio were fit and available for selection.
"Declan Rice and Elliot Anderson trained with us today," Tuchel said. "They had minor issues after the game, but everyone is available for us apart from Reece James."
On Saka's fitness, Tuchel added: "He's now fully free of discomfort and is ready to go."
Anderson's future had also attracted attention amid reports linking him with a possible big-money move from Nottingham Forest to Manchester City, but Tuchel insisted transfer talk would not distract the squad.
"The reality is that transfers are going on and big transfers take their time," he said. "We understand things are going on. We will facilitate the transfer but after the match, not before."
Despite Panama already being eliminated, Tuchel warned his players not to underestimate their opponents.
"They are a well-trained team," he said. "They can play under the underdog tag without pressure."
Ghana boss Carlos Queiroz has dismissed suggestions that his team could lose to Croatia for a better 2026 FIFA World Cup draw, insisting they will play only for victory.
Didier Deschamps returns to France's FIFA World Cup camp on Saturday after briefly leaving the squad to attend his mother's funeral, as Les Bleus continue their impressive run at the tournament.
Egypt coach Hossam Hassan has moved to ease concerns over Mohamed Salah's fitness after the captain was forced off during Friday's 1-1 World Cup draw with Iran, saying the forward will be assessed ahead of the knockout stages.
Argentina coach Lionel Scaloni has confirmed Lionel Messi would start on the bench when the reigning FIFA World Cup champions face Jordan in their final Group F match at Dallas Stadium on Saturday.
