The Southampton defender admitted Japan had benefited from two controversial penalty decisions after a 1-0 victory over Oman in Abu Dhabi at the weekend sent the Blue Samurai into the last 16.
"We have to be more clinical," Yoshida told AFP after Japan's second straight win in Group F.
"We should have scored two or three more. Our performances are still not good enough clearly."
Genki Haraguchi converted from the spot after 28 minutes after tumbling theatrically under little or no contact.
Oman then had a legitimate penalty shout waved off after Yuto Nagatomo appeared to use his hand to block a shot just before half-time.
"We were a little bit lucky with the penalty we had -- and the one not given as well," shrugged Yoshida.
"But the most important point for us was to get through the group."
Nagatomo likened his lucky escape to Diego Maradona's infamous intervention against England at the 1986 World Cup, expressing relief that the VAR (video assistant referee) system is not used during the Asian Cup group stages.
"It hit my hand," he admitted. "It's lucky there was no VAR because it probably would have been given. Happily the 'Hand of God' was with us today."
A new-look Japan were similarly unimpressive in beating Turkmenistan 3-2 in their opening fixture, but were missing two-goal hero Yuya Osako for the Oman clash due to a thigh injury.
The Werder Bremen striker was sorely missed against the Gulf side, with Takumi Minamino squandering a hatful of chances.
"It was tougher than we thought it would be," conceded Yoshida.
"If we had taken our chances we wouldn't have struggled so much late on. But I'm confident we will find our form."
Japan, who reached the knockout stage at last year's World Cup, have won a record four Asian Cups but appear to lack the quality of Iran and South Korea at this year's tournament.
"We can't expect to dominate every game," said Japan coach Hajime Moriyasu, who is unbeaten in seven matches since taking over after the World Cup.
"It was important to get through the group and we have done that. The key will be to grow into the tournament game by game."
But Yoshida insisted Iran, who are purring, and a South Korea side boosted by the arrival of Tottenham forward Son Heung-min will be the teams to beat -- along with holders Australia.
"I saw Iran at the World Cup, where they played really well," said the 30-year-old.
"I was really impressed. And of course Australia, South Korea -- they're always the most difficult opponents.
"We have to make sure we're ready for them, we have to improve."