Striker Rowan Vine grabbed an early goal for Rangers but they could not build on their advantage and NK Karlovac ran out 3-1 winners.
"I've learned a lot more about my players," said Magilton.
"No-one likes losing, least of all me, but the lads have got some more game time in their tanks and that's the most important thing at this time of year.
"I've learned a lot throughout the tour and we'll highlight those things to the lads over the coming days and weeks. Everything is geared towards August 8 against Blackpool."