Freeman limped out of the 1-1 home draw against Bolton on Tuesday night after 29 minutes with an ankle problem.
Tests have shown no serious damage but the npower Championship club expect to be without the 20-year-old for between three and four weeks.
"Kieron left the ground on crutches on Tuesday night and having had a scan, it looks like he'll be out for a few weeks," said Rams boss Nigel Clough.
"It's a blow for him as the injuries gave him a chance to impress and stake a claim, but thankfully he'll be back in a few weeks."
Derby were already missing left-back Gareth Roberts through suspension and centre-backs Jake Buxton, Mark O'Brien, James O'Connor and Shaun Barker due to knee problems against Bolton.