The 27-year-old German striker has signed for the Gunners this summer already and will join an attacking lineup that includes Robin van Persie, Gervinho, Theo Walcott and Marouane Chamakh.
The writer of Fever Pitch, High Fidelity and About A Boy believes the former Bayern Munich forward can be a success at the Emirates Stadium.
"He can become the biggest Arsenal star, the biggest star in our capital. And our hero," he told German publication Kolner Stadt Anzeiger. "I have watched all his goals for Cologne that he scored this season.
"When it became more probable that he would sign for Arsenal, I watched him closer. I'm fully convinced by him."
Hornby also predicted Germany will triumph at Euro 2012 over Spain and group competitors Netherlands, Denmark and Portugal.