The veteran Portuguese coach opted to rotate his squad for the 4-1 weekend LaLiga win home win over Rayo Vallecano as Yan Diomande made his first start since joining the club over the summer for a record £120M fee.
The Ivory Coast international completed 72 minutes at the Estadio Bernabeu, before being replaced by Guler, who went on to set up Kylan Mbappe for the hosts fourth goal in added time.
Mourinho was positive in his post-match assessment of Diomande's progress, but also calmed fears over Guler's place, as he confirmed squad rotation is just inevitable.
"Arda plays a big role when he's on the pitch. I'm 100% sure if Diomande hadn't started, the first question would have been, 'Why isn't Diomande playing?'
"You only see quality on the bench. Changes will happen game by game. Arda has a very positive influence on the team. When he came on, he made a real difference."