The missing player is:
the German winger, Serge Gnabry
and the result of the game between Portugal and South Korea will be:
a 4-3 win for Portugal.
To find these out, we must deduce that the scorelines here are generated by...
... (i) identifying the player in the image, then (ii) counting up the number of letters in their surname (or most commonly used name) that also appear in the name of the participating countries, ignoring any later duplicate letters.
This can be seen in the six known results:
Nathan AKE: EcuAdor vs nEth[e]rlAnds : 2-2
Gareth BALE: EngLAnd vs WALEs : 3-3
Lionel MESSI: argEntIna vs MExIco : 2-3
Kingsley COMAN: frANCe vs tuNisiA : 3-2
Luka MODRIC: CanaDa vs CROatIa : 2-4
NEYMAR: bRAZil vs cAMERooN : 2-5
Note that in all cases we are using the common name of the country in English.
With this rule, we can work out that the score of the game between Portugal and South Korea should be:
Cristiano RONALDO: pORtugAL vs sOuth k[o]ReA : 4-3
And by looking at the squads of Costa Rica and Germany we can find the missing player to be:
Serge Gnabry, of Germany
Serge GNABRY : costA Ric[a] vs GeRmANY : 2-5