The Union advanced to the Concacaf Champions League semifinals Tuesday night at Subaru Park, finishing their series against Atlanta United with a 1-1 tie for a 4-1 aggregate win. Here are some observations on the game.

Jamiro Monteiro stars

From the sixth minute when he played a sumptuous through ball to Sergio Santos, he was terrific: six chances created, 18-of-26 passing, four duels won, and eight recoveries.

But all the good work he did was overshadowed by a yellow card he earned for a tackle in the 51st minute. That gives him two bookings in the Champions League — his first was in the game at Saprissa — which means he’ll miss the first leg of the semifinal against Club América or the Portland Timbers. The winner of that series will be decided Wednesday night in Mexico City.

» READ MORE: How to watch Wednesday's Concacaf and UEFA Champions League games

Strong defense early

The Union turned the press up high early on as they looked to put the game away at the attacking end of the field, creating nine chances and taking 12 shots in the first half. But they were also stout on defense. Despite having nearly 70% of the possession, Atlanta was able to get off just one shot in the first half hour. Santiago Sosa’s brilliant goal for Atlanta in first-half stoppage time was United’s second shot of the game.

Leon Flach steps up

He had a big task starting for the suspended José Andrés Martínez, and answered the call. Though he isn’t as much of a ballhawk as Martínez, he nonetheless recorded six recoveries, two tackles, two clearances and one interception, and won seven duels. He also completed 19 of 24 passes, including 16 completions in the attacking half of the field.

» READ MORE: José Andrés Martínez waits to find out whether he'll get an extra suspension from MLS

Kacper Przybylko scores again

It took a long time for the Union’s goal to come, but when Kacper Przybylko finally fired past Atlanta goalkeeper Brad Guzan in the 88th minute, it was his fifth goal in the Union’s Champions League campaign. That’s a big statement by the team’s top striker. Now he can focus on getting his first goal of the year in MLS play, after going scoreless through three games this year.

Back line concerns

Wagner took the brunt of a bad collision with Atlanta’s Jake Mulraney in the 38th minute. He was able to come back into the game a few minutes later, but took a while to get back to full speed. Fortunately, he seemed to be fine during the second half.

Matt Real will also be suspended for the semifinal first leg after getting booked in this game and the Saprissa series first leg.

In the short term, the key worry should be about centerbacks Jack Elliott and Jacob Glesnes. Elliott has played every minute of every game this year so far, and Glesnes has played every minute but one. With Stuart Findlay sidelined by a quadriceps injury, they likely won’t get much rest soon. The Union visit the Chicago Fire on Saturday (1 p.m., 6ABC), then host the New England Revolution next Wednesday (7:30 p.m., PHL17).