The Seleccao went ahead through a close-range Nani goal but second-half strikes from Jermaine Jones and Clint Dempsey left Jurgen Klinsmann's side heading for the Round of 16 with a game to spare.
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However, Cristiano Ronaldo whipped an inch-perfect cross into the box in the last minute of stoppage time for Silvestre Varela to head home, preventing USA from immediate qualification and Portugal from imminent elimination.
But Portugal must win its final game against Ghana and hope that Germany defeats the USA, and overturn a five-goal difference between the two teams in the process. Bento is realistic that his team looks set to be dumped out of the tournament on Thursday.
"It is a result that is bad, that's obvious," he told RTP. "It does not put us out of the World Cup but it leaves us in a very precarious situation. We started well, with getting the goal, but then had great difficulty in holding off the right flank of USA. We tried to change this situation in the first half and the remainder in the second but we could not.
"The 2-2 result was representative of what we did with our last effort, an effort that was great considering the difficult conditions in Manaus. Now we have to play the last game and exhaust all of the possibilities we have, though there are few at this time."