Caligiuri, now of Eintracht Braunschweig, played with Muller at both Greuther Furth and Mainz, with the 25-year-old having struck five times for the latter already this season.
Muller heads the Bundesliga scoring charts, level with Stuttgart's Vedad Ibisevic, but missed out on a call-up to the Germany squad for this week's FIFA World Cup qualifiers, despite making Joachim Low's side for their close-season trip to the United States.
The Braunschweig midfielder does not believe it will be long until Muller is back in contention under Low.
"If he keeps playing the way he is, it’s only a matter of time before he gets called up again," he told the Bundesliga's official website.
"But for now, he’s just focusing on this season with Mainz and whatever else happens is a bonus."
Muller played for Furth for five years before moving to Mainz in 2011.
Caligiuri feels Muller's spell at the Trolli Arena, which was spent exclusively in the 2.Bundesliga helped him grow as a player.
"Nico has become much more mature in every sense and is now playing at a very high level in the Bundesliga," he continued.
"He has been through highs and lows, which is unusual. In Furth, he was naturally disappointed when he wasn’t playing much early on. He matured a lot as a result and is now able to deal with setbacks a lot better.
"That speaks a lot for his personality. It makes him strong."
After finishing 13th in the Bundesliga last season, Mainz have made an impressive start to their campaign this time around.
Thomas Tuchel's men have won three of their four matches and sit in fifth place.
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