Portland improved to 3-3-7, winning its second straight game and third in its last five.
Chivas USA dropped to 2-6-4.
Former Chivas midfielder Jorge Villafana helped create the first goal when he ran down the left flank in the 65th minute and crossed the ball to Adi for a shot from 10 yards.
Adi scored in the 71st minute on a breakaway.
Chivas USA's Bobby Burling was ejected in the 84th minute after a rough challenge on Adi.
The Timbers held a 65% to 35% advantage in possessions in the first half and had five shots on goal to one for Chivas USA, forcing Dan Kennedy to make three saves.
Chivas USA's scheme shifted in the second half, but Portland capitalized on the counterattack, with Villafana recording his first assist in only his second start with the Timbers.