Mermis was recalled from AHL Toronto on Monday.
The Maple Leafs returned Fraser Minten to the minors in a corresponding move. Mermis will provide the big club with an extra defender after Oliver Ekman-Larsson (lower body) suffered an injury in Saturday's 4-3 win over Edmonton. If Ekman-Larsson can't play Tuesday versus Calgary, Philippe Myers will probably draw back into the lineup after being a healthy scratch in Saturday's victory.