The former Hibs and Rangers hitman joined English League One outfit Peterborough United last week after being shipped out on loan again by Forest, following a disappointing spell at Ibrox last term.
And City Ground boss Aitor Karanka has warned 22-year-old Cummings to shape up at Posh.
He said: “We want him to show what he can do. Sometimes it is good for a player to realise that it is not always easy.
“He went to Rangers at the start of the year and he was scoring goals to begin with.
“But by the end Rangers did not want him, so he came back here.
“Maybe his expectations were higher. But at the end of it he has to go to League One.
“Life in football can be difficult and you have to work hard to take the right steps forward.”