He cost you the game...

There are four key phases to effectively help your goalkeeper process a mistake that cost your team the game: during the match, after the final whistle, the day after the game, and throughout the week. During the match, when your goalkeeper makes a big mistake on a decisive goal, words often have little impact. From the sideline, there’s not much you can do except encourage them to lift their head and "stay in the game." Yelling at them will have the opposite effect. At that moment, it’s up to the goalkeeper to handle the situation as best as they can and show mental resiliency—which is something you’ve ideally worked on regularly during training sessions. In such situations, it’s mainly their teammates who can provide comfort. A few encouraging words from them can help ease the disappointment. This is when the team must support their goalkeeper instead of turning their back on them. Once the game is over, you move into the second phase.

Saying, "It’s not a big deal," is actually reminding them that the team lost because of them…

In the locker room, don’t bring up their mistake. Instead, show your support through gestures—a pat on the shoulder, a reassuring look—indicating that you’re right there with them. Words, in the heat of the moment, are rarely helpful. They can even be harmful, as they may be misinterpreted. Something you think is supportive might carry a completely different meaning to someone still shaken by their mistake just minutes earlier. Even saying, "It’s no big deal, it’s just a mistake," unintentionally reminds them that their error cost the team the game. It’s better to approach them a couple of hours after the game, perhaps during the ride home. By then, they might feel ready to talk about what happened. Let them take the lead. If they’re not ready, don’t push. Let it go and wait until tomorrow, which I believe is often the best time to address the situation. During warm-ups or stretches, try bringing up the game again.
 

Wait until the next day to discuss it, allowing the goalkeeper to express their feelings, process the situation, and receive guidance.

Initially, focus on discussing the game overall. This provides an opportunity to exchange opinions, allow the goalkeeper to open up, and reset on a positive note. From there, gauge the situation. If you sense that your goalkeeper wants to revisit their mistake, don’t hesitate to address it. The primary goal is to put the incident into perspective. Help them understand why the mistake happened, even if it means recreating the play during warm-ups to simulate the situation in a controlled environment. However, be careful not to center the entire session around the error from the previous day. This would give the mistake too much weight and could put additional pressure on your goalkeeper heading into the next game. Moreover, never bench them for the upcoming game, unless the mistakes have been recurring over several weekends. Finally, throughout the week, you might briefly touch on the error during a drill or discussion but keep it brief and constructive. Avoid overemphasizing the mistake, as it could hinder their confidence.