"We created chances enough in the first half, and then, to be honest, it has been my mistake because I did the change before I saw the extra time," a relieved-looking Maresca told reporters.
"When I saw 12 minutes, I saw that probably was too early to make that kind of change. But fortunately, we won the game, and we are happy."
With the clock ticking down, and Chelsea ahead through Enzo Fernandez's 50th-minute header, Maresca replaced attacker Cole Palmer and Fernandez with defender Tosin Adarabioyo and midfield anchor Kiernan Dewsbury-Hall, packing his back line.
The change allowed Spurs to pour forward in their search for an equaliser during the long period of stoppage time, which came after two long VAR checks in the second half which ruled out goals for both sides.
Maresca was full of praise for the way his players soaked up the pressure to seal the win which gave Chelsea some fresh momentum in their hunt for a top-four finish and a place in next season's Champions League.
"If you want to become a team, I think you need also to learn and to play in the way we played in the last 10 minutes: win a game in a dirty way," the Italian said.
Maresca said the return of Senegal striker Nicolas Jackson from injury had given his side more shape, thanks to his incessant pressing of the Spurs defence: "For sure, with Nico we are a different team."