The Argentina international, who has scored 62 goals in 80 appearances in the elite European competition, suffered a thigh injury in Saturday's 2-0 win over Almeria and is expected to spend up to three weeks on the sidelines.
In Messi's absence, coach Gerardo Martino has to figure out a replacement and Fabregas is eager to take on the role of lead forward.
"Messi is the best player in the world, he makes a difference, but he won't play [against Celtic]. We can't do anything about that," the former Arsenal midfielder told reporters on Monday.
"Without Messi, we will all have to take more responsibility. We have to step up, don't feel his absence and play football without him.
"I might have more offensive responsibilities. I will try and help the team as a striker or whatever other position I play.
"Now I feel confident, in good shape and the coach trusts me. I am in a good moment. Maybe I need to be more consistent."The Catalans suffered a 2-1 defeat at Celtic Park at this stage in the competition last season and, while Fabregas is aware of the difficulties they will face on Tuesday, he feels the team has learned from their mistakes.
"We’ve learned our lesson from last year," the 26-year-old continued. "They had two chances and scored two goals. It could be a very similar game to last year, especially as it was successful for them.
"We are not upset about last year, we played a good game but must not give the ball away, maximise our chances and score goals.
"If you follow the Champions League you know coming to Celtic Park will always be a tough game.
"I played here with Arsenal as well and it is difficult. The Celtic fans are always behind their team. They take a corner and it feels like a goal."
Neymar remains another option to replace the injured Lionel Messi, and Fabregas admits the young Brazilian would be more than up to the task.
"He’s got so much talent and he’s like lightning in tight spaces," he added. "He’s a star, but in the future he’s going to be even bigger.
"He’s got the world at his feet and we’ll see how he develops. It’s a tough game to face the challenge of replacing Messi, but if anyone is ready to do it then it’s Neymar.
"Pressure doesn’t affect him. I think he’s ready mentally. We really need the wide players."