Nyassi also set up forward Eduardo Sebrango - on trial with the team - with a through ball in the 10th minute, but Sebrango's shot missed wide right.
"We really picked up from our last game against Houston," said Nyassi. "We made sure that we were compact defensively and moved up well as a unit. It was unfortunate that we weren't able to finish our chances."
Brazilian midfielder Felipe had a chance at goal in the 39th minute, but his shot missed to the right of the Timbers goal.
Montreal's Sinisa Ubiparipovic curled a free kick into the box in the 78th minute. Forward Evan James redirected the ball just over the net.
"I found that we matched them physically," said Impact head coach Jesse Marsch. "We even set the tone, and there were some good moments soccer-wise.
"If we can keep doing that, then we feel like the good plays will come out."
After the game, the Impact played a 45-minute game against the third division L.A. Blues of the USL Pro league, which also ended in a 0-0 draw.