Mariano has been strongly linked with a loan move away from Real Madrid ahead of the winter window, but it is understood that the striker's recent form has forced Los Blancos boss Zinedine Zidane to rethink the situation.
The Dominican Republic international scored his first La Liga goal in Real Madrid's 3-2 win over Deportivo La Coruna last weekend and also recently struck a hat-trick in the Copa del Rey.
Leganes are one of a number of clubs that have been linked with the 23-year-old, but Garitano 'does not think' that the striker will be allowed to leave the Bernabeu next month.
"I don't know if he will leave - I don't think so," Garitano told reporters.
Mariano was a regular scorer in Real Madrid's Castilla side before being promoted to the first team before the start of the 2016-17 campaign.