A sentence can have a maf'ul bihi even when no separate noun appears as the fa'il.
نَصَرَ الْمُسْلِمِينَ — he helped the Muslims.
- the past verb نَصَرَ carries an understood “he” as its fa'il;
- الْمُسْلِمِينَ are those who received the help, so the word is the maf'ul bihi;
- because it is a sound masculine plural, ya is its sign of nasb.
Do not call the nearest noun the fa'il automatically.