Alonso struck the winner against Southampton on Saturday.
"It is a very good weapon for us," Courtois told chelseafc.com. "Cesc and Willian can also hit them well, so for a goalkeeper it's hard to know who is going to shoot.
"Marcos was surprisingly shooting from there; I think he saw the gap next to the wall and he has the quality to score that. We're very happy.
"I only saw it quickly on the screen, but I think the wall jumped the other way, so that's why the ball got past on that side.
"Obviously it's a bit annoying for a goalkeeper if you put your wall and it jumps the other side and it leaves a gap. You see the ball very late and it's hard to save. But from our point of view it's very good."
On Alonso's influence, Courtois didn't hold back.
"Marcos plays so many games on that wing and I don't think people can imagine how hard that job is, because it's always attacking-defending, attacking-defending,"
"You don't stop, you're running a lot, and he's playing almost every three days. He does very well for us and he's improved very much throughout last season and this season as well.
"His quality of free-kicks is amazing and those situations in football nowadays are so important, so if you can score from that it's very good."