The 27-year-old suffered the latest in a series of serious injuries when he damaged the anterior cruciate ligament in his right knee in January.
Rossi recovered in time to be named in Cesare Prandelli's provisional 30-man squad, and was involved in the 0-0 warm-up draw with the Republic of Ireland on Saturday.
However, Rossi suffered the heartbreak of being excluded from Prandelli's final 23-man party for the marquee tournament in Brazil, which was announced on Sunday.
"Everyone says I'm out of shape: ask anyone the results from the tests during the week and the game. You will be shocked," Rossi wrote.
"Challenges? Fear? What a laugh. But now the important thing is to support Italy. Forza Azzurri."
It marks further disappointment for the American-born forward, after he was overlooked for Italy's 2010 squad, with former manager Marcello Lippi later admitting he made a mistake by not taking him to South Africa.
