The 28-year-old scored 15 goals in his first season with Sundowns in 2015/16 as part of a lethal frontline that included Khama Billiat and Keagan Dolly, who has since moved to Montpellier in France.
However, Castro's profile waned afterwards and he aims to revive his career at Chiefs following a January switch from the Brazilians.
The Colombian hitman, who scored on his debut for Amakhosi against Baroka FC after coming on as a substitute last month, has started the last two games for the club.
Castro believes he is close to nearing match fitness.
“Leo is back after I didn't play for a long time at Sundowns. So I was feeling like I'm losing my profile and my confidence in the last month at Sundowns,” Castro said.
“So now I feel better, I work hard every day to improve my profile again. The coach and my teammates are giving me this confidence to show that I'm slowly coming back again. I'm training hard to make the Kaizer Chiefs family very happy.”
Castro had a good game against Cape Town City last week where he created the only goal scored by Ryan Moon. He feels the goals will come if he stays on the pitch for the full 90 minutes.
“I'm getting better, now I feel like I can finish the game because most of the time the opportunities and the chances are happening in the last 10 minutes. So if I stay more time in the game I will score two or three goals maybe.
“But I'm feeling well and the team is doing well and I'm happy.”
Castro recognises similarities in his game with the in-form Moon, who has scored five goals in his last six games for the title contenders.
“The coach wants me to play with Ryan and I said to him you are putting in the same guys, we are playing the same. I told him that we are not the fast players where we can run at defenders, but now we have confidence to do the things inside the field. It doesn’t matter who is playing or the style of the player.
“If you are doing well and staying in the right position and right mentality, things will happen in the field.”
Steve Komphela's side next host Bloemfontein Celtic at FNB Stadium on Sunday looking to trim Downs' five-point lead at the Absa Premiership summit. Both sides have nine games left in the league.