All the hard work for the English champions was done with a 5-0 win in the Portuguese capital three weeks ago as City reached the last eight for the fifth consecutive season.
“It’s a good sign,” said Guardiola. “The last decade, step-by-step the club grew up. Every season playing this competition, qualifying for last 16 and now in the best eight teams in Europe.”
City are on course for a fourth Premier League title in five seasons, but the Champions League has so far remained out of reach despite the billions invested by the club’s Abu Dhabi owners.
Guardiola knows his legacy in Manchester will be judged on whether he can finally make City champions of Europe.
However, he believes the achievement of just becoming a regular contender should not be overlooked.
“Every year when we qualify for the Champions League I celebrate it that night because I know how difficult it is,” he added.
“When you qualify for last 16, I celebrate it. Now it’s time to congratulate everyone, focus on the Premier League and see who we get in the draw.”
A much tougher test will lie ahead in the last eight as Sporting set out at the Etihad just to avoid another embarrassing scoreline like the one they suffered on home soil.
“After the result of the first game the tie became practically impossible,” said Sporting goalkeeper Antonio Adan.
“The team was better defensively, and they didn’t play with the same clarity either. We leave satisfied.”
Guardiola handed a European debut to teenage right-back CJ Egan-Riley with a number of defenders out through injury, illness or suspension.
Rodri, Kevin De Bruyne and Riyad Mahrez were also rested from the start, but City should still have won the game comfortably.
Raheem Sterling wasted the best of the home side’s chances as the England international failed to lift the ball over Adan from Phil Foden’s sensational through ball.
After an uneventful first half, City thought they had claimed the lead through Gabriel Jesus early in the second period.
Mahrez, introduced at the break, ran into the box and slid the ball into the Brazilian on the left.
Jesus smashed in from a tight angle but the goal was disallowed for offside following a VAR review.
The state of the tie was summed up when Guardiola took the chance to give 36-year-old goalkeeper Scott Carson his first Champions League appearance since 2005, replacing Ederson 17 minutes from time.
Carson was called into action to bravely block from Pedro Porro with Sporting’s best chance moments later.
Mahrez, John Stones and Sterling all missed chances late on to break the deadlock, but Sporting salvaged some pride by holding out for a clean sheet.