Union goalkeeper Zac MacMath was forced to make a diving save in the eighth minute on a looping shot by Kris Boyd, who joined the Timbers during the offseason from famed Scottish club Glasgow Rangers.
The Union did not record a shot until the 18th minute, when another newcomer - Colombian forward Lionard Pajoy - shook off a defender and struck a low drive wide left from long range.
Pajoy started up front with Josue Martinez, which relegated former No. 1 draft pick Danny Mwanga to the bench.
Boyd and MacMath tangled again in the 21st minute, as MacMath charged out of his net to clear the ball away from a hard-charging Boyd.
Four minutes later, Boyd crashed into Carlos Valdes as both players leapt for the ball, and the collision left Valdes sprawled on the ground holding his head. Valdes returned to the field a few minutes later, but appeared groggy.
Slowly, the Union was able to build up more possession of the ball. In the 31st minute, Michael Farfan worked his way open in the center of the field about 25 yards from goal; he had space to shoot but opted to pass toward onrushing Pajoy. His pass went awry, though, and the Union's best chance to score in the first half fizzled out.
The Union got on the board first, in the 51st minute. Freddy Adu was fouled in possession of the ball about 35 yards from goal, and midfielder Gabriel Gomez smashed a swerving free kick past Portland goalkeeper Troy Perkins. The ball took a slight deflection off Timbers defender Andrew Jean-Baptiste, but not enough for it to be ruled an own goal.
Within 3 minutes, Portland tied the game off a free kick of its own. Jean-Baptiste headed a service from Jack Jewsbury down and toward the net, and the ball slipped under MacMath as he sprawled on the wet turf.
MacMath redeemed himself quickly, though, making a double-save and two more diving stops over the next 4 minutes.
Boyd scored the winning goal in the 66th minute, thumping a header past a stranded MacMath off a cross from Alhassan. Ten minutes later, Alhassan sealed the victory with a spectacular chip over MacMath and a cluster of Union defenders from an acute angle.