Dempsey put the Sounders (7-12-3) up, 2-1, in the 37th minute, tapping a right-footer from the top of the 6-yard box into an open net. Christian Roldan lofted a pass from near midfield to Jordan Morris near the top of the box. Morris, as goalkeeper Joe Bendik charged, dropped a soft pass to Dempsey for the finish.
Seb Hines gave Orlando City (5-6-11) a 1-0 lead in the seventh minute.
Dempsey hammered home a cross from Tyrone Mears in the 14th to tie it, and capped the scoring — on a goal nearly identical to his second of the game — in the 48th minute.
Orlando City had its three-game unbeaten streak snapped.
Portland played most of the match with 10 men, after Diego Chara was shown a red card in the 12th minute for apparently elbowing Sporting midfielder Benny Feilhaber in the face after the two players got tangled up.
On the day United moved closer to signing Paul Pogba in potentially a world-record transfer, its most high-profile offseason signing so far made an immediate impact in the English season’s traditional curtain-raiser at Wembley Stadium.
Having a quiet game by his high standards, Ibrahimovic met Antonio Valencia’s right-wing cross by leaping high and directing his header in off the post.
Eleven weeks after scoring an extra-time winner for United in the FA Cup final at Wembley, Jesse Lingard returned to the stadium to put his team ahead in the 32nd minute with a solo goal.
Jamie Vardy equalized for the Premier League champions in the 52nd.
