The Scotland international collided with teammate Ben Turner just before the half-hour mark and was taken off the pitch covered in blood after scoring the opener in the 10th minute.
Bluebirds Malky Mackay still doesn't know how the injury occurred but said Miller would have needed a hospital visit and will be in doubt for Tuesday's Carling Cup fourth round clash with Burnley.
"The one person you don't want to run into in our team is Ben Turner," he said. "I don't know how it happened, but it was like something out of comic cuts.
"I still don't know how it happened, but it was a 5ft 10in player running into a 6ft 5in giant. Ben was okay, but Kenny had to have stitches in a nasty cut on his head.
"The doctor will have to check him out before we decide whether or not he will be alright to play on Tuesday night against Burnley."