The Brazilian, who is on loan from Wolfsburg, found the top corner in the early stages of the second period, before Neymar levelled 20 minutes from time for the home side.
"My goal was a golazo and a very important goal. I'm very happy," he told reporters.
"Goals give me confidence and they're important in making me comfortable. It'll be good for the future. I always dedicate my goals to God. The last few games haven't been easy for me.
"My goal put the team in front, so I was happy. It's good to play, to feel important, to score. The celebration was for the entire team."
Diego is due to return to his parent club at the end of the season.