The 36-year-old attacking midfielder was also wondering what he had done wrong.
Asked whether he had ever got an explanation from former manager Ryan Nelsen on why he wasn't playing, De Rosario replied: "No, not really."
"When I did speak to the coach, it was all '(You're) doing well, long season,'" De Rosario said Monday after training with the national team. "That's what made me more frustrated because as a player, as a senior player, you just like things straightforward. I wasn't getting any response in that direction."
DeRo started three of Toronto's first five league games. The former MLS MVP did not get his next start until Saturday, Toronto's 26th game of the season.
That latest start was at the behest of Greg Vanney, who replaced Nelsen as manager on Aug. 31.
Vanney wasted little time calling De Rosario, who was in camp with the Canadian team, after the coaching change.
"That was huge from a coaching standpoint," De Rosario said.
And when De Rosario left the Canadian camp early Friday to play for Toronto against the Philadelphia Union the next day, he sat down with Vanney and heard how the new coach saw him fitting him into his plan.
"That was another positive," De Rosario said. "Hopefully we can keep those lines (of communication) open because whether I'm a starter or coming off the bench, I still think I have a lot to offer in terms of information about other teams playing in the league so long . . . I know what it takes to win in this league.
"Right not we're lacking a little bit of the mentality part of it. You can see with the goals that we let in. And it's very important that we battle in Chicago the next game coming up and grind out a result and build off of that confidence and steam forward."
De Rosario is a 14-year MLS veteran with 336 appearances in the league with five different teams.
When asked about De Rosario, Nelsen always cited him as a model pro who was helping younger teammates at practice and in the locker-room. But he said it was hard getting game time with Nigerian international Bright Dike and former English Premier League forward Luke Moore vying with De Rosario to get playing time behind Jermain Defoe and Brazil's Gilberto.
That has led to speculation that De Rosario's re-signing with the team was GM Tim Bezbatchenko's idea and not Nelsen's.
De Rosario said his sole focus, after Canada's friendly Tuesday against Jamaica, is to get TFC into the playoffs.
Toronto (9-11-6) slipped from fourth to seventh in the Eastern Conference standings on the weekend after losing 2-0 to Philadelphia and seeing rivals post wins.
Toronto has lost three straight, all by shutout, and only won three of its last 15 games (3-7-5), dropping 31 of 45 possible points during that run.
Vanney's team has eight games remaining in its bid for a first every trip to the playoffs. The top five teams in each conference advance to the post-season