Alcacer converts again to continue Monaco woes
Leonardo Jardim's Monaco slip to the foot of the table as their miserable start to the season continued -- the principality side have also won just one of their first eight league games of the campaign and sit in the Ligue 1 relegation play-off place.
The best chance of the first half fell to the hosts just seconds before the interval, but Diego Benaglio was quick to dive to his right and turn Marius Wolf's low shot past the post.
But Monaco goalkeeper Benaglio was injured in making the save, and had to be replaced by Danijel Subasic.
Dortmund coach Lucien Favre threw on Dane Larsen at half-time in place of Wolf, and the decision paid off in the 51st minute.
English teenager Jadon Sancho was the creator, showing great vision to slip a perfect pass into the path of Danish youngster Larsen, who took a touch and drilled home.
The Bundesliga outfit were gifted an opportunity to double their advantage with 21 minutes to play as Reus tumbled to the ground under a clumsy tackle by centre-back Kamil Glik.
Alcacer, making his full debut for Dortmund, contrived to rattle the crossbar, though, as he attempted to lash the spot-kick down the middle.
But the Spanish international quickly made up for the miss, skipping clear of goalkeeper Subasic and firing into the net to score his fourth goal for the club in just three appearances.
And the 20-year-old Larsen did well to get free down the left-hand side in injury-time, and crossed low for Reus to slot into the corner at the back post.
Dortmund top the early Group A standings from Atletico Madrid on goal difference, after the Spanish side's 3-1 victory over Club Brugge at the Wanda Metropolitano.