The teenage duo are expected to be utilised in Lampard's first-team squad after returning to action with the U23 side last week.
But Lampard wants to see them gain more fitness before returning to be considered for selection.
"They are fit but not match fit," said Lampard ahead of Tuesday's Champions League clash with Valencia.
"Callum is getting extra work in and with Reece, he played in the Under-23s and they may need another game in the Under-23s in them before they are ready for action. N'Golo similarly.
"Although not to play in Under-23s, he will have more training to do just because the injury has been bothering him."