The Brazilian has been one of United's most consistent performers this term and will look to help maintain his side's six-point lead at the top of the Premier League when they host Sunderland on Saturday.
The 22-year-old has also played more games - 21 in all competitions - than any other player in United's squad this season and thinks Fabio's loan move to QPR has helped him develop.
Asked what was behind his form, Rafael said on Manchester United's official website: "I think this is just a natural development, it's bound to happen when you play more games and have more experiences.
"My daughter has also helped me, [being a father] has settled me down a lot. When you're a dad you have to be mature.
"Also, I think splitting from Fabio has been a good experience. At first I was sad, but I've come to see that it's made me grow up a lot. I've had to change and be more responsible."