Ronaldo swapped Real Madrid for Juve in a stunning €112 million deal at the start of the season and the five-time Ballon d'Or winner justified the investment as he led the Serie A holders into the Champions League quarter-finals.
The 34-year-old forward scored a record-equalling eighth Champions League hat-trick to see off Atletico Madrid 3-0 in Turin on Tuesday and 3-2 on aggregate.
Ronaldo has netted 24 goals in all competitions in his first season with Juve – 19 of those coming in Serie A – and team-mate Costa revealed his initial concerns about the dressing room becoming unstable have been proven wrong.
"Believe it or not but when Cristiano Ronaldo's arrival was being talked about, I asked 'what will the dressing room be like?'" Costa told YouTube channel De Sola.
"We all have to love each other as team-mates because we're all looking for a place in the team - but Cristiano won everyone over very quickly.
"He's a simple, hardworking guy who likes to win. Juventus were looking for a player with his mentality, so he's given us a lot.
"When he arrived. I thought 'the best player in the world has joined, right?'. As soon as he arrived, the club improved in every aspect. His contribution has been noticeable.
"He scores goals every week, he’s brought more fans to the stadium and we hope to win things with him so we make history with him by our side.
"He's a stand-up guy, he's hard-working and he's always with us."
Juve – who will face Ajax in the Champions League quarter-finals – travel to Genoa on Sunday and are 18 points clear at the Serie A summit.
Ronaldo meanwhile was named in the Portugal squad for the first time since the 2018 World Cup on Friday with the country no doubt hoping his club form will translate to the international stage ahead of Euro 2020 qualifiers.
So good was Ronaldo's performance on Tuesday in the Champions League that even Lionel Messi surprised by his efforts.