Ronaldo picked out the bottom corner of the net 62 minutes in at San Siro after a give-and-go with Miralem Pjanic to bring up his landmark strike, shortly after having a shot from close range closed down by Stefan de Vrij.
Juventus boss Allegri was pleased with the way his side, without key men such as Paulo Dybala, Mario Mandzukic and Sami Khedira, battled back to earn a share of the points in the Derby d'Italia clash.
Radja Nainggolan had opened the scoring seven minutes in with a stunning long-range strike and Inter ended the first half on top, but the visitors responded well after the break and could have returned to Turin with all three points, with momentum shifting after Ronaldo's crisp finish
"It’s important to have someone like Ronaldo, because before the goal he made a mistake on another chance and his reaction to that changed his performance and consequently the whole game," Allegri told DAZN.
"We did risk going 2-0 behind, but then it was a good second half performance. It wasn't easy - Inter are a strong side and we were down to the bare bones. We've got to compliment the lads, as we ended on a high."
Juventus sealed their eighth successive Serie A title with last weekend's 2-1 win over Fiorentina, but that achievement has been clouded somewhat following their quarter-final defeat to Ajax in the Champions League.
Allegri concedes that too many errors are creeping into his side's game, having now won just one of their last five matches in all competitions.
"Against Fiorentina last week, it was similar to the Ajax game, in that we conceded insane counter-attacks," he said. "Inter did have three counters tonight too when it was 1-1, but our defenders did well to get back.
"We have to improve in our passing, because once again we made so many mistakes.
"I asked the lads to pass it more cleanly and we wasted several scoring opportunities. In any case, I found it a very enjoyable match. It was a whole other game in the second half."