Manchester City full-back Joao Cancelo is the player making way, while Dalot being picked as his replacement is reward for the young Red's impressive performances at the recent Under-21 tournament.
Portugal were runners-up in that competition, narrowly losing 1-0 to Germany in the final earlier this month.
In a statement published on Sunday, the Federation of Portuguese Football (FPF) revealed that Cancelo had tested positive for COVID-19 in Budapest, where his national side will take on Hungary in Group F on Tuesday.
The FPF added that every other player in Fernando Santos's 26-man squad, including United playmaker Bruno Fernandes, tested negative for the coronavirus.