Feldman (4-6) was called to take the mound after scheduled starter Roy Oswalt was scratched due to back stiffness. He held the Red Sox to one run on seven hits, didn't allow a walk and struck out five.
Napoli's two-run blast in the sixth handed Texas a 6-1 lead and Josh Hamilton knocked in a pair of runs as American League West-leading Texas took the opener of a three-game set.
Boston starter Felix Doubront (10-5) was roughed up for six runs on eight hits over five innings.
Jarrod Saltalamacchia homered for Boston, which dropped its fourth straight and fell below .500 (48-49).