B767 First Officer reported fuel imbalance and possible fuel leak after departure. Flight returned to departure airport.

Narrative

Shortly after takeoff from ZZZ; we got a message in our FMC 'insufficient fuel' EICAS message. We had confirmed all of our fuel requirements during our preflight and checklist procedures. The captain suggested loading in new updated winds into the FMC which the winds didn't correct the problem. shortly after we also noticed there was a 1000# fuel imbalance with the left tank lower.At this point we suspected a potential fuel leak. We balanced the fuel initially and started a timer to measure the rate of any further loss. The captain asked me to run a suspected fuel leak checklist in the Flight manual and we decided our situation didn't quite fit the description in the checklist because the fuel levels were at times correcting themselves (although never fully to our planned Fuel over destination). We contacted the dispatcher working our flight and Maintenance Control. We decoded we would monitor and make a decision at our coast out point. Upon reaching the coast out; the projection was showing our fuel over destination was already 4000less than planned and getting worse. We decided the safest course was to return to ZZZ. We followed the company diversion guide and checklist and coordinated with ZZZ Oceanic and dispatch to turn around and go back. The return back to ZZZ was uneventful