The wingers have been side-lined with hamstring problems, forcing them to miss the games against Krasnodar, Everton and Wolves.
They host West Ham on Monday – but that game might come too soon.
“Hakim and Callum will hopefully be training by the end of the week," said Lampard. “Whether they are fit or not for West Ham, I'm not sure."
But Lampard believes tiredness has been another factor.
“There could be some fatigue," he said. “I hate to say it after a defeat because it could sound like an excuse, but even the first 60 minutes when we were the better team we didn't have a zip about us.
“We have travelled up to Everton, come back and up to Wolves, but everyone is in the same boat on that front.
“At the moment we have a couple of injuries in areas that mean I can't make too many changes or rotations in certain areas of the team."