England top the group on four points, level with Ghana, with Croatia third on three and Panama bottom without a point.
Tuchel confirmed that James has not travelled with the squad and said the right back would definitely miss the game, though he hopes he can return later in the tournament.
"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.
"No-one could see that coming, Reece felt good and was in good shape. We would love to have Reece, he's a good player and we will look for solutions. That's what we do."
Tuchel said James could be available for the round of 32, but conceded the timeline was "tight".
There had also been concerns over Rice and Saka, while Anderson had minor issues after England's 0-0 draw with Ghana. Tuchel, however, said the rest of the squad were in contention.
"Declan Rice and Elliot Anderson trained with us today," he said.
"They had minor issues after the game but everyone is available for us apart from Reece James. We will take a decision tomorrow."
On Saka, Tuchel added: "He's now fully free of discomfort and is ready to go."
Anderson has also been the subject of transfer speculation, with British media reporting a possible big-money move from Nottingham Forest to Manchester City. Tuchel said England would not allow it to become a distraction before the Panama match.
"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."
Tuchel warned that Panama, despite having already been eliminated from the tournament, would not be straightforward opponents.
He described them as "a well-trained team" and said they "can play under the underdog tag without pressure."