2000 E250 5.4L new engine issues

I just had an engine replaced and a transmission rebuilt. On the way home from picking it up I noticed there was not alot of power and is running a little rough. It is kind of like it is being held back. I called my mechanic and he said give it a couple days for the computer to receive everything. So it has been a few days and still having the issue. Check engine light came on with a p0174 code which says too lean bank 1&2. I called my mechanic back and he says he is pretty sure the oxygen sensors have gone bad and to bring it back and they will replace it. It just seems a little odd that they were fine before I took it in. I just wanted some opinions on here. TIA!