Grant looked drawn when asked to assess his side's upset.
After seeing Tykes striker Kayode Odejayi hit the winner, he also laid some of the blame on the Oakwell pitch, and said: "It's a big disappointment. It's a quarter-final and we lost.
"First of all I need to congratulate Barnsley. They played very well today and showed good spirit.
"For us it's not a pitch we can play quality football on. It was not good in the first half and the second half we started very well.
"We played quick, we created a lot of chances and then, like it happens sometimes in football, we conceded a goal and we couldn't score.
"I don't think we thought this was an easy game. We knew they beat Liverpool away, so we knew what to expect.
"I wanted both (Lampard and Drogba) to play, but they were injured, so they couldn't, but we played with a team that was good enough to win the game."
Grant added: "I don't know if I'm shocked, but I'm very disappointed. You can say so yes, since I came to Chelsea, this is the most disappointing day."