Real are already qualified for the next phase.
Mourinho said: "It's not a high-risk game where if you lose you are out, but Zagreb will be difficult because in the first leg we played with our best team and we won by the narrowest of margins (1-0).
"They are the Croatian champions and they have 10 points more than the second-placed team in their league.
"I have to make changes in my team because I have very tired players, not because the opponents will be easy."
He added: "We are group leaders with 12 points, with four wins, and we want to be first in the group. We don't know if being first will help us, but it gives us something of an advantage.
"We want to play to win tomorrow, score goals, and entertain the fans in the process."