Barca's recent performances have been marked by defensive lapses and a lack of punch in attack and they need to overturn a 2-0 deficit when they host AC Milan in a Champions League last 16, second leg on Tuesday.
Before then, they will be looking to bounce back from last weekend's league reverse at Real Madrid and protect their 11-point lead over second-placed Atletico Madrid when bottom side Deportivo La Coruna visit the Nou Camp on Saturday.
"It's obvious that we need to give a bit more," Messi said at a promotional event in the Catalan capital.
"We know what the problems were in the defeats, it's something we have talked about in the dressing room and it has to stay there," added the four-times World Player of the Year.
"We have been losing recently and we know what we have to do to change.
"A key match is coming when we can react and make that change."
Barca's league defeat at Real came four days after their arch rivals dumped them out of the Copa del Rey, ending the club's hopes of repeating 2009's historic treble under Pep Guardiola.
Guardiola, who will take over at Bayern Munich next term, handed the reins to Tito Vilanova at the end of last season but Vilanova has been struck down by illness and is undergoing treatment in New York.
His assistant Jordi Roura has taken charge until he is well enough to return.
"It has affected us a great deal and I think it has taken its toll when we have been out on the pitch," Messi said.
"But this is a strong dressing room and we cannot lay the blame there," he added.
"Coming from behind (against Milan) is what we all want. We know it will be difficult but we have faith.
"If they could do it to us there we can do it here. We know that if they score it will make things harder and that Milan has some very good players but we have our chances."
- Sports & Recreation
- Deportivo La Coruna
- AC Milan
- Champions League