The Saints threw away a one-goal lead to lose 3-1 at Everton on Saturday and have now let in 18 goals in their opening six matches this campaign.
"First half was not so good and second half we were a little bit better, but we still need some improvement, especially defensively from the crossing," said Yoshida.
"We made too much mistakes. Too easy to lose the ball in the midfield. We have to change these mistakes. Against Arsenal and [Everton] we lose three quick goals and too easy I think.
"In defence we should not change after we lose one goal. Offensively, we should keep the ball a little bit more longer."
Saints remain in 17th place in the Premier League.