Portugal beat the Netherlands in last year's final to claim the title, but Croatia failed to make an impression in the same tournament as they picked up just one win from their four matches.
Portugal triumphed in the inaugural season of the UEFA Nations League, beating Switzerland in the semi-finals of the competition before recording a 1-0 victory over the Netherlands in the final last June.
It would be fair to say that the nation's main focus is on preparing for next summer's European Championships, but they will still be keen to make another impression in this competition.
Fernando Santos's side have won seven of their last eight matches in all competitions, including a 2-0 success away to Luxembourg in their last game in November 2019.
Portugal qualified for the European Championships courtesy of a second-placed finish in Group B, and they are the defending champions, having overcome holders France in the 2016 final.
A Selecao will continue their UEFA Nations League campaign against Sweden on September 8, while they are pencilled in for a glamour friendly against Spain on October 7.
Croatia, meanwhile, will also be present at next summer's European Championships, having finished top of Group E, picking up 17 points from their eight matches during a strong qualification campaign.
Unbeaten in their last six competitive matches, Vatreni will also enter this match in good form, and there is no question that they are a team to be taken seriously on the world stage.
Zlatko Dalic insisted ahead of this match that the UEFA Nations League is "important" but was keen to stress that the nation's main focus is on their upcoming World Cup qualifiers and the European Championships.
Croatia, who have lost four of their previous five games against Portugal, will continue their UEFA Nations League campaign away to France on September 8 before taking on Sweden on November 11.
As mentioned, the 2018 World Cup finalists failed to make an impression in the 2018-19 competition, picking up just one win from their four matches against Spain and England.
Portugal Nations League form: WDDWW
Portugal form (all competitions): WWWLWW
Croatia Nations League form: LDWL
Croatia form (all competitions): WDWDWW
Portugal boss Santos is expected to send a strong XI onto the field, with Cristiano Ronaldo set to win his 165th cap, and he will be looking for his 100th goal for the national side.
Ivan Perisic is expected to play despite representing Bayern Munich in the Champions League final, though, and there will still be plenty of familiar faces in the away side's XI.
Chelsea's Mateo Kovacic is likely to join Inter Milan's Marcelo Brozovic in the middle of the park, while Bruno Petkovic is expected to get the nod through the middle, having found the back of the net on five occasions in just eight appearances for the national team.
Former Liverpool defender Dejan Lovren should also feature for Croatia as part of a back four.
Portugal possible starting lineup:
Patricio; Semedo, Fonte, Dias, Guerreiro; Neves, Danilo, Fernandes; Ronaldo, A Silva, B Silva
Croatia possible starting lineup:
Livakovic; Jedvaj, Lovren, Caleta-Car, Barisic; Brozovic, Kovacic; Perisic, Vlasic, Rebic; Petkovic
We say: Portugal 2-1 Croatia
This is a very, very difficult match to call between two strong teams. Both will feel that they can cause issues in the final third of the field, but we are finding it hard to look past Portugal on home soil, ultimately backing them to shade a close game in Porto.