The Sweden coach saw his side undone by Cristiano Ronaldo's 82nd-minute header in the 1-0 first-leg loss on Friday, with the Real Madrid man overshadowing fellow Ballon d'Or hopeful Zlatan Ibrahimovic to give his side a narrow advantage heading into Tuesday's return fixture in Solna.
Ronaldo's strike was a fair reflection of a second period dominated by Paulo Bento's men, but it could have been a different outcome had Sweden taken one of the first-half opportunities they created, most notably when Johan Elmander connected with Mikael Lustig's centre in the early exchanges.
But Hamren admitted that Portugal's performance after the break proved too difficult to handle.
"They (Portugal) pushed more in the second half and we were forced to play too many balls in the air," Hamren said.
"The goal we conceded was a bit sloppy by (the front) post. I'm always disappointed when we lose, I would have been really happy with the draw.
"We must work hard (in the second leg). I hope we will play better at home.
"After this we must win the next game at all costs. We will do everything we can to win."
