James lasted only 27 minutes of Sunday's 2-0 loss at Everton, forced off with a hamstring problem.
"Reece felt something in his hamstring. For sure he's injured, but we don't know how serious it is," said Pochettino, whose side are down in 12th place.
"Of course for us he is an important player, one of the best full backs in the world. We are disappointed because when you believe you can build something and a player like him who should be really important for us can't help the team.
"We go back and assess again in the next few days what is going on with his hamstring."
James was not the only injury concern after the defeat at Goodison Park which left Chelsea 18 points behind leaders Liverpool and far closer to the bottom three than the top four.
Goalkeeper Robert Sanchez had to be replaced by Djordje Petrovic while full back Marc Cucurella also went off injured.
"Robert Sanchez suffered a problem at Manchester United, it was an issue with his knee," Pochettino said. "He felt something during the second half, that is why he asked for the change. We must hope it is not a big issue.
"That is the same for Marc Cucurella, who twisted his ankle. We hope that these issues are not big. These are the circumstances we have been dealing with since the start and it is too much for a young team that needs to have all the elements together if we want to build something solid."
Chelsea have made their worst start to a Premier League season since 2015-16 when they ended up 10th.
