The Argentine explained he though it would be like the one he kicked against The Netherlands in the World Cup
Javier Mascherano explained in an interview in the TyC Television why he indicated ter Stegen the direction of the penalty kicked by Kun Agüero in the match against Manchester City. He sensed Kun would kick it the same way the one he kicked against The Netherlands: "It wasn't easy to kick this penalty. Yesterday we talked about that with Kun. You never know which direction Agüero will choose. It was intuition because in a similar situation, against The Netherlands, he kicked to the same side. I though the same could happen."
The Argentine indicated the direction in a subtle way with his finger. Ter Stegen explained after the action that he also received the indication of Rafinha, who pointed to the opposite side.
Here you can see the image of Mascherano before the penalty: