Rosa arrived at Ajax in January and produced a good performance in the recent clash against PSV to gain credibility with the Amsterdam team's fans.
With an assist to his name, he was the best player on the pitch according to Flashscore's ratings system in the 2-0 win that saw Ajax go nine points clear at the top of the Eredivise.
He was previously at Valladolid in Spain, who are battling against relegation, and was linked with a number of Brazilian clubs at the start of the year before ultimately heading to the Dutch capital.
"I'm happy with the possible interest from Brazilian clubs, but it was speculation. There were only concrete offers from Ajax, and I'm very happy to be playing for a club like this," he told Flashscore.
Eight years ago, Rosa visited Amsterdam with his mum, Ana Teresa Ratti, who is also responsible for managing his career. One of their visits was to the Ajax training centre. There, Lucas was impressed by what he saw.
"I told her that one day I would play for this club. Things happened in my favour to make this wish come true and it was easy to make the decision to sign with them," says the full-back, who was developed by Palmeiras and arrived in Europe at the age of 18 to play for Juventus, where he played for the under-19 and under-23 teams before moving to Spain.
The negotiations between his former and current club took place when Valladolid were in a better position than they are now, at the bottom of La Liga, and Rosa says the possibility of relegation didn't influence his decision to leave.
"The team was coming off the back of some good games, and we'd taken a breather in the league. I'd already played for Valladolid in the second division, I wouldn't have had a problem staying there, even with the chance of going down. The move came about because of Ajax's good offer," he said.
He has since played six games for the Dutch side and has seen his game develop under highly-rated Italian coach Francesco Farioli.
"He likes 1v1 marking, intensity in defence, and that requires greater responsibility in this area. I've realised that I've developed a lot here and my dream is to make the national team," he said.
Against PSV, Rosa showed his potential and made a big impression on the fans, who may be seeing him in an Ajax shirt for a long time, given that his contract is for four years.
"I did well in a six-point game, I was able to show what I'm here for, and I'm happy to have helped out," he said.
"I've adapted well to the style of play in the league, which is very different from LaLiga. I'm going to keep looking for my place, even after a mid-season transfer and with the challenge of adapting."
This good run of form makes him want more, preferably in Europe.
"I see my path in Europe over the next few years, I'll try to take it one step at a time so that one day I can make the national team," the 25-year-old said.
"The Brasileirao has grown, it has players of international renown, and it's a tournament that gives important visibility, but my focus is on Ajax. That's where I want to show my football."