Wrong gearbox installed?

I sent my vehicle into the garage to get the turbo replaced. The mechanic just told me it was all finished and as he was resetting the computer sensors, he noticed one was missing as there was no slot for this particular sensor. The gearbox was replaced just before I bought the car in early 2022. He told me the wrong gearbox was fitted and it was a big problem. I'm wondering if this is even possible, and just how big a problem can this sensor be?