You don't have to because that's what the opamp is for. But I don't know the specs of the internal ADC on MSP432, or how it is set up in MSP432. You're most likely giving it a weird reference voltage. If you have a nice test bench, you should sweep the current and look at the results. If the opamp and the multimeter has a constant difference but otherwise following the current change accurately, you have an offset error and it's actually not a big deal, you can even fix it in digital if you don't care about using the full scale range. If the ADC is giving weird data regardless you need more info to debug.