Kieran Lee and Gary Hooper look set to face more weeks on the sidelines after missing much of last season through injury.
Jos Luhukay said both players had initially tried to return to first team training when pre-season began but they have suffered setbacks.
The manager said Hooper, who has been out since December with a hip injury, had reverted to training individually last week.
Lee, who was ruled out for most of last season with a recurring hip problem, has also had 10 days not training with the first team squad.
Luhukay said: “These two players I think will take time to come back 100 per cent.
“But I hope that Kieran and Gary will make progression in the next few weeks so that we have more possibilities and choices for positions.”