Diop stopped all five shots he faced and finished the match despite sustaining a right leg injury in stoppage time that left him unable to put any weight on it.
Montreal was playing its first match since the international break. CF Montreal hung on for the draw despite playing the entire second half with 10 men after Zorhan Bassong received a red card.
D.C., which beat Inter Miami 1-0 on Saturday, recorded a clean sheet for the third straight match. The visitors failed to convert on any of their 27 shot attempts.
After an opening few minutes in which both sides struggled to push the attack beyond midfield, D.C. began to find its offensive groove.
Adrien Perez split Montreal's defenders for a chance from inside the box in the 14th minute but was denied by a diving save from Diop.
Perez was at it again four minutes later, taking a lob pass from Paul Arriola from midfield to the top left corner of the box for a shot, but Diop got a piece of it to deflect it over the net.
Montreal found its legs late in the first half, getting chances from Ahmed Hamdi and Mustafa Kizza but couldn't find the back of the net.
United outshot Montreal 10-2 in the first half.
Montreal ended the half down a man as Zorhan Bassong was assessed his second yellow card for getting his hand on the ball, leaving the club to compete the rest of the match with only 10 players.
United picked up where it left off in the second half but couldn't beat Diop.
A wide-open Paul Arriola got off a quick shot in the 58th minute, but Diop was ready.
Montreal had one of its best opportunities of the night when midfielder Djordje Mihailovic's shot from the top of the box rolled just wide of the net.
Diop injured himself when putting the ball back in play after the save on Robertha. He was tended to by trainers before gutting it out and opting to stay in the game.
D.C. goalie Bill Hamid was credited with one save.