The two coaches have a shared history, having worked together at Bayern Munich and with the German national team.
Following Rohl's confirmation as Rangers' new head coach, Flick, now at the helm of the Catalan giants, sent a congratulatory text.
Rohl shared the details of the message during a recent interview with Sky Sports.
He said: "He [Flick] wrote me a message [saying], 'Good luck and all the best', and that he's very happy I joined this club now.
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"And that he'll cross his fingers.
"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's first outing as head coach of Rangers is set to take place tomorrow away to Norwegian side Brann in the Europa League.
The new head coach inherits a team that recently drew 2-2 with Dundee United, a match for which under-19s head coach Stevie Smith took interim charge.
Rohl has indicated that he will be implementing some changes to the team's lineup for his debut match.
He said: "We will have some changes in the starting XI tomorrow.
"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."