While City have a 100% home record, the Blues have picked up just one point on the road since the opening-day win at West Ham.
City are still riding high in 3rd place, but Pompey are also going well and lie in 6th having lost just twice this season.
"We started off with some good performances away, and we know that it's something we have to work on," said Sven. "It's a concern, but you should not look at the bad things all the time.
"If you ask the players and the fans, they would say they are happy when they look at our position in the table. If we could improve away, and keep up our standard at home, then things would be much better but every game is difficult.
"This is a big game for us as well as them," he added. "Portsmouth have done very well so far this season.
"They are a physically strong side, with a lot of pace as well. They have quality all through the team and a lot of experience so I'm expecting a very difficult game."
It will be the first time City play against Sylvain Distin, who left the Blues in the summer after making over 200 appearances for us.
And although he's settled in well on the south coast, Sven believes he hasn't been missed.
"He's strong, a very good footballer for a centre-half, and from what I hear, he's doing very well for Portsmouth," he said.
"Everybody here also still talks in good terms about him. But we've not missed him because we have some very good centre-halves here, with Micah Richards getting better and better."