The Brazil-born forward played his first games for Ukraine this month, but Portugal and Luxembourg raised doubts over his eligibility.
UEFA is investigating whether Júnior Moraes is eligible to play for Ukraine after Portugal and Luxembourg lodged complaints about him featuring in Euro 2020 qualifiers against the two nations.
Moraes, a Brazil-born striker, is enjoying a fine season at club level with Shakhtar Donetsk in Ukraine, the country in which he has played much of his senior career.
He received his first call-up to the Ukraine team ahead of recent qualifying matches against Portugal and Luxembourg and played in both games, having reportedly been granted a Ukraine passport in March
However, both opponents have questioned his eligibility to play for Ukraine as a naturalised player, as FIFA regulations state a player must have "lived continuously for at least five years after reaching the age of 18" in the relevant country.
The longest continuous period Moraes has lived in Ukraine appears to be four years and seven months, having played for Metalurh Donetsk and Dynamo Kiev between July 2012 and February 2017, when he moved on loan to Chinese side Tianjin Quanjian.
UEFA confirmed Luxembourg and Portugal lodged a "protest" to the Control, Ethics and Disciplinary Body, which will look into the matter on a date yet to be confirmed.
Moraes is the third Brazilian to represent Ukraine after Edmar and Marlos, the latter of whom is his team-mate at Shakhtar.