if your mechanic suggested to replace the injector without first diagnosing if it is infact the problem then i would be going elsewhere
what i did was take it to my mechanic, he then pulled out the injector, found the broken o-ring, looked at the injector (a nismo 555cc one in this case), replaced the o-ring, and pressure tested the fuel system to (i think) twice the required working pressure and all was sweet.
IMO, buying a second hand injector is cheap, but no point if the problem isn't identified with certainty.....