There are always two ways to analyze a performance like Portugal's against Croatia and project what the rest of the World Cup might hold for the national team.
Firstly, the negatives: the team struggles defensively, disorganizing during defensive transitions. This weakness has become more apparent as the caliber of opponents increases, especially when playing with a numerical disadvantage in midfield, as seen between the 62nd and 81st minutes. This opened up gaps that, by sheer luck, didn't result in goals. Some players haven't replicated their club form for the national team, notably Bernardo Silva, Bruno Fernandes, and even Vitinha. João Cancelo oscillates between effective attacking plays and poor defensive decisions. Furthermore, the central defensive pairing is below the standard Portugal has enjoyed this century (Fernando Couto/Jorge Costa, Ricardo Carvalho/Jorge Andrade, Pepe/Rúben Dias, to name a few).
Secondly, the positives: much like Rui Patrício in Euro 2016, Portugal has a point-saving goalkeeper in Diogo Costa. Nélson Semedo is increasingly reliable and deserves the starting right-back spot. Nuno Mendes is improving after a season hampered by physical limitations. Gonçalo Ramos hasn't lost his goal-scoring instinct despite limited minutes in the group stage. Cristiano Ronaldo scored again (from a penalty, yes, but the goal disallowed by mere centimeters was executed with rare precision). By winning the game without their captain on the field, Roberto Martínez earned credit both externally and internally.
Now comes Spain, European champions and one of the tournament favorites, even with Lamine Yamal still not at 100 percent. In their four matches, they've displayed better football than their Iberian counterparts. However, recent history will be forgotten if the national team can replicate in 90 minutes what they achieved in 45 against Croatia: dominating possession and stealing the ball.
Portugal has no other way to beat La Roja – they are not a transition-based team, nor are they physically imposing, nor sufficiently lethal from set pieces. So far, the chemistry expected from a midfield that promised classical music-like performances hasn't materialized, but there's still time. We always have the Nations League final as a benchmark, one of the games where João Neves played at right-back (in the first half). And even then, it worked out well...