Ricciardo has struggled throughout his first F1 season with McLaren but was on form all weekend at Monza, finishing third in the sprint before controlling proceedings from the front in the grand prix.
Ferrari had been able to take advantage of McLaren only scoring meaningful points through Lando Norris in the fight for third but Leclerc warned his team that Ricciardo's performance would make things difficult in the championship run-in.
"Days like this shows they are really on it," Leclerc told Sky Sports F1 after McLaren's one-two finish.
"I am very happy for Daniel, it is great to see him perform as he would have done last year. He is an incredible driver and it is great to see him back at 100 per cent.
"But for us, it is bad news if he keeps performing like this. It is pushing us to try and be perfect every weekend and that is exactly what we will try to do in the next few weekends."
Ferrari pace not good enough for podium in front of Tifosi
Leclerc had made the most of a free pit stop under safety car conditions to jump to second behind Ricciardo after Lewis Hamilton and Max Verstappen collided on lap 26.
The Monégasque could not hold onto his position, however, and lost out to Lando Norris and Valtteri Bottas, although Leclerc was handed fourth after Sergio Perez was penalised for overtaking him off-track.
When put to him that Ferrari's pace just simply was not enough to earn a podium in front of the Tifosi, Leclerc replied: "No it wasn't.
"It was a shame. The pace was there but the only thing is we were very vulnerable in the last sector and on the first straight so that made it very easy for the others to overtake.
"I made a lot of lap time in the second sector but the thing is they were recovering more lap time in the last sector and the first sector. That is how we got overtaken.
"It is a shame but in the end, I gave it absolutely everything."
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