The pair worked together at Bayern Munich and with the German national team. Flick, now in charge of the Catalan giants, dropped Rohl a text after he was confirmed as the new Ibrox head coach.
Rohl revealed all in a recent interview with Sky Sports, stating: "He [Flick] wrote me a message [saying], 'Good luck and all the best', and that he's very happy I joined this club now. And that he'll cross his fingers.
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"It's always great to have such a message from such an experienced coach. And it's also good to know that when you need a call, then you can call him, and you can speak about football and sometimes personal and private things."
Rohl to make changes for first Rangers match
Rohl's first game in charge of Rangers comes tomorrow evening away to Norwegian outfit Brann in the Europa League.
His new team drew 2-2 with Dundee United last time out, with under-19s head coach Stevie Smith taking interim charge of the first team for the match.
Rohl has confirmed that he will make some alterations to the lineup for his maiden outing: "We will have some changes in the starting XI tomorrow," he revealed. "But after the first two days, I cannot overload the players, especially coming into a Europa League game.
"I am looking forward to the game. Over the last couple of days, I have seen some good things from the squad.
"We have to turn the negative energy into positive energy, which, from the day I met all the players, I saw that smile on the players.
"Over the last two days, the players have been working hard, and you will see that in the game tomorrow. It will be a fight, but we are ready for it."