The United States is the single largest recipient of FDI in the world. Among the countries that invest the most in the United States, Japan stands in the third position, with the UK and Canada coming in the first and second position respectively in 2018. (See: https://www.selectusa.gov/servlet/servlet.FileDownload?file=015t0000000LKSn)
Japan's share of the total FDI in the United States is 11.2% whilst that for UK is 13.7% and Canada is 13.5%. In terms of FDI Stock dollar value Japan has invested $489 billions in the United States as of 2018. These are formidable numbers and hence the decision by the US to exclude Japan from the whitelist would certainly impact Japan adversely.