Calderwood said: "He's very sore at the moment, but we'll just have to wait and see the results of the scans. Right now it looks quite a nasty one.
"He'll certainly be missing against Blackpool on Tuesday night, and that's something we had not bargained for."
The 1-0 win over the Bantams suggests that Forest are better able to cope with the pressure of being promotion favourites.
Last season they struggled spectacularly, before launching a late play-off push after sacking Gary Megson.
Calderwood added: "It's nice to get three points on the board. If they are three points out of 92 it will be very helpful. If it's three points out of 62 they will be rubbish.
"We know we can do better, and that's particularly with the ball going forward. But we showed a lot of resolution without the ball and worked hard to fill holes and pockets."