The former Wolverhampton Wanderers man played 29 times for Posh last season, but has missed the club's first two games of the new campaign.
Ferguson has admitted that the 21-year-old is not yet fit enough for first-team football.
"Mendez didn't come back to the club anywhere near fit enough. Gone are the days when you can come back overweight and use pre-season to catch up," Ferguson told Peterborough Today.
"Nat's a good player with the potential to be very good, but he knows what we expect and it's up to him to force his way back into the side.
"We will play a reserve team match in each of the next few weeks and to be fair the attitude of the whole team in the game this week was very good."
Peterborough travel to Notts County on Saturday.