The Germany international quickly won over the Real fans after joining the club from Werder Bremen before the 2010/11 season and developed lethal on-field partnerships with forwards Cristiano Ronaldo, Karim Benzema and others.
This season, however, the return to form of Kaka after a long injury layoff has seen Ozil slip behind the Brazil playmaker in the pecking order and he appears a shadow of the midfielder who sparkled for Germany at the World Cup.
"It's mostly because the team have improved. We are better than last year and it's more complicated to claim a place in the starting line-up," Mourinho told a news conference on the eve of Real's Champions League Group D game against Olympique Lyon.
"No one can expect to always play the full 90 minutes."
Mourinho suggested Ozil could even be facing competition from Portugal's Fabio Coentrao.
"Last season Ozil was the only number 10 and now we have Kaka and Fabio Coentrao, who can also play there," he added at the Stade Gerland.
Ozil is expected to start on Wednesday, though, with Kaka missing out through injury as Real look to reach the knockout stages by beating Lyon, who they thrashed 4-0 two weeks ago.
