Not sure if this has been analysed already, but Injustice 2 should be over or really close to 1 million units sold.
MK8D first month was 460k. We know from Reggie that MK8D second month is larger than the individual SKUs of Injustice 2, or larger than half of total Injustice 2 sales. As a result, Injustice 2 < 2*MK8D second month, and therefore MK8D > 460k must apply, since Injustice 2 could not be above the sum of MK8D first + second month otherwise. So, Injustice 2 > 920k, and it's definitely possible that it is over a million. A side effect of this is that MK8D is also quite close to a million, so great sales for both series!
Edit: This obviously does not take into account the fact that there are more expensive limited editions for Injustice 2, which could skew the analysis.