The 44-year-old, who took the reins from Pep Guardiola at the end of last season, is currently in New York undergoing cancer treatment, but is expected to return to Catalonia on March 25.
Barca have suffered a dip in form recently, losing twice to bitter rivals Real Madrid, as well as the first leg of their Champions League last-16 tie with AC Milan.
"Nowadays the coach is really important," Messi told TVC Sports. "Since Tito hasn't been here we've really noticed it.
"We respect Jordi Routa because he's our stand-in coach and he's trying to help us win everything we can.
"But our boss was Tito and it was a blow when he went."
The Catalan giants are 13 points clear at the top of La Liga.