Complex sentence: The Buccaneers were allowed to use Carter's roster exemption on another player from the NFL Europe League who was not already affiliated with an NFL team, with the proviso that, eventually, that exemption would have to be assumed by only one of the two players.
Split into simpler sentences: The Buccaneers were allowed to use Carter's roster exemption on another player from the NFL Europe League who was not already affiliated with an NFL team. Eventually, that exemption would have to be assumed by only one of the two players.