Chelsea defender Ricardo Carvalho admits they cannot afford any early slip-ups in their Champions League group.
Carvalho, a winner of Europe's top competition with Porto, said: "They are not easy teams to play against, we know more or less Valencia because we played last year, and Schalke are a good team and had a good season. Rosenborg I don't know so much, but the first game is at home so that is good.
"I've been to the semi final twice with Chelsea and had good experiences. Of course I won with Porto, that's my best football moment so far. It's a competition some players can't get, and its difficult to get it, it's a pleasure to have that medal.
"Last season, we didn't have the luck with us because we missed penalties, so let's see. When we start we are thinking about winning the Champions League because it's important for us, but you must have a little bit of luck.
"After 90 minutes in the group stage, if you don't play well you can't go out so you have a little bit of room, you need that.
"It's important in this competition to start winning, at home you have to win and try your best away. The sooner you get the points the better."