"Of course, we are not happy with this result because our goal for every game is to take three points," Petrovic told chelseafc.com.
"We were in control in the first half but in the second half, we conceded two goals and then we had a fight on our hands. Credit to Brentford, they played a good game but we are not happy because our goal was to win this game.
"Now, we must focus on the next game and putting it right against Newcastle. With the game being on Monday night, we have a lot of time to recover and focus on what we need to do to get the three points there."
On a rollercoaster week, which included an FA Cup win against Leeds, but also their Carabao Cup final defeat to Liverpool, Petrovic said: "We feel the pain from this result because we want to win every game and it's off the back of a tough week, following the defeat in the Carabao Cup final.
"After that game at Wembley, we needed to get back to winning ways and we found that extra motivation and energy against Leeds. We had a good result and we wanted to do that again against Brentford.
"These are things we can learn from. The final was a good experience even though it wasn't the end result we wanted. We are a young team and it was a good lesson for us, to improve on next time.
"We have big games coming up in both the Premier League and the FA Cup and there is still so much for us to achieve this season."