So are you expecting Injustice 2 to do more than 500K because that's the type of second month MK8D is probably going to be looking at. After all it was only available for a few days at the end of April and managed nearly that amount. Right now that larger Switch stock is nearly gone in the US. But no matter how limited - I expect at least 375K Switches sold to the US for May NPD.
MK8 on the Wii U 8 sold
470,000 physical and digital copies in June 2014 NPD. It was topped only by Watch Dogs second month and at the time WD was possibly one of the biggest games that had come out on the PS4/Xbox. It was the biggest IP at the time and since than I think only new IPs that have outsold it are Destiny and Overwatch.
From the venturebeat article:
June 2014 was the second month for both WD and MK8, and thus far we know that MK8D's launch has outsold the launch of any other MK in the US. Even if Switch stock is somewhat limited for the demand the system is generating I still think MK8D will have a strong month that would be able to clear 500K.