I had this very thing happen recently. Running on centre tanks but somehow developed a fuel imbalance in the wing tanks big enough to generate a fuel configuration Eicas. The QRH does not consider this scenario; it says crossfeeds on, low side off (which of course does nothing when you're running on centre tanks) The options are:
1) follow the QRH and wait until the centre tanks are empty for the wing Pumps to start working and the crossfeed to occur - which could be several hours.
2) turn the centre pump off corresponding to the high side, allowing the high side to burn some fuel and balance out, then turning the centre pump back on - which takes a few minutes.