2001 Mitsubishi Galant Fuel pump replacement

That's one heck of a sentence. There's a hundred things to look at. You haven't said what your mechanic has checked so far. If he replaced the generator he also had to disconnect the battery. The issue could be as simple as the Engine Computer has to relearn "minimum throttle" before it will know when it has to be in control of idle speed. Until that is done you may have to hold the accelerator pedal down 1/4" to start the engine. The assumption, of course, is the mechanic has already checked the basics like spark and fuel pressure.