The Argentine went down under a challenge from Nathanial Clyne and had to be taken to hospital for an MRI scan.
Mancini said: "We don't know at the moment, we need to wait maybe two days for the scans. We hope it's not serious but at this stage I don't know. I am worried, yes. This can happen in every game, it is clear that it is important that you are also lucky through the year but today it's a problem.
"Usually the knee is always dangerous, this injury was maybe so strange. If it is a serious injury, we will probably change our plans [to bring in a replacement]. Maybe he is out for a week, or three or four weeks, or after the international break."