With two Group G games to play, victory for the Hoops would ensure a last-16 berth while a score draw would be enough if Barcelona beat Spartak Moscow.
The Scottish champions have been buoyed by their recent win over Barca and an away win in Moscow, and Swede Mjallby said: "We are in a great position.
"There is more pressure on Benfica because they really need to win the game. But it is a difficult game, it's a Champions League match away from home and we really respect Benfica, we think they are a really good side.
"But the confidence is there. We've shown we can play away from home so why not go out and nick that vital point?
"They are probably going to go all out to try and win the game. They are going to be strong at home, a good side going forward, well organised.
"But we have picked up a lot of confidence from our away games and we know if we defend really well we can get something from this game."