The France attacker faces his former club Borussia Dortmund in the Champions League this week.
Dembele said, "During the preseason I was very well. Afterwards I got injured and I was out for a month. When I returned, I had some physical problems. Now I have to fight for my place because we have a great team and there is more competition than in past seasons.
"There are many, very good forwards, with a lot of quality, and we're motivated to be even better.
"Since my arrival at Barça three years ago I have improved in my preparation for games. I go to the gym and warm up properly. Barça are a great team, with great players and that makes you want to win to give everything."