Posh were on the losing end of a 3-1 encounter with Southampton on Tuesday night but are guaranteed to be playing their part in English football's second tier next season.
Ferguson said: "It is our biggest achievement as a club by miles but it is a very different feeling to winning promotion. This is more about relief than joy.
"We have stayed up because we were outstanding in the first half of the season. We haven't been great lately but always knew the effects of having such a short break after winning the League One play-offs last summer would catch up with us.
"Our lads deserve great credit for what they achieved - none more so than Gaby [Zakuani] who is a brilliant example to young players - but we really want to win our last home game against Watford on Saturday for the fans."
Peterborough are 18th in the Championship and eight points clear of the relegation zone.