Derby spent most of the summer chasing the pair, before eventually getting deals over the line.
However, both Evans and Marriott have had to watch from the sidelines for the Rams so far, with Lampard being cautious with them both.
And Lampard claims that the new additions have had to play catch-up on their fitness ever since arriving at Pride Park.
He said: "The problem is, when you sign players quite late, some of the moves are quite protracted and the player spends a week or two not really training.
"George and Jack have both had that scenario, but they are getting towards full fitness now although they haven't played any games, so they will need a bit of time.
"It is important we get those players we are talking about minutes, so that will be at the forefront of my mind [for Derby’s cup match in midweek].
"There will be changes to see what the players who come in can do.”
Derby take on League Two side Oldham Athletic tomorrow, in a match where Marriott and Evans could feature for the first time.
Derby are under pressure to win, after two rather underwhelming performances so far this season.
The Rams opened up their season with a slightly unconvincing 2-1 win over Reading, before they were battered by Leeds United on Saturday.
Marriott’s potential inclusion tomorrow, in particular, will excite Derby fans - as he netted 33 times last season for Peterborough United.