Telling a teammate “you’re better than this” only works if you’ve already shown you believe in them.

The same sentence is encouragement or an insult depending on what came before it. If somebody knows you rate them, it reads as a reminder of a standard you both accept. If you have never said anything positive, it reads as a telling-off dressed up as a compliment. Those words carry whatever history you have with the person, and nothing else.

Build the credit before you need it by telling people what they are good at while nothing is going wrong. Then keep the harder line for when it is true. If you have not earned it yet, say the plain version instead.