After Bill Hamid's save, D.C. United pushed the ball up field and Arrieta slid the ball under Evan Bush, who had come out to the corner of the 18-yard box to attempt an interception of Chris Rolfe's lofted pass.
Three minutes later, United's Chris Pontius hit the crossbar from just outside the 6-yard box.
United was without its two leading goal scorers from last season. Luis Silva has a hamstring injury and Fabian Espindola is on suspension.
Montreal, in its fourth MLS season, has lost three of four openers.
United won the 2014 Eastern Conference regular-season title behind Coach of the Year Ben Olsen, but lost to the New York Red Bulls in the conference semifinals. Montreal was one of three clubs to finish with a MLS-low six victories.
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