The 1.94-metre striker scored just one goal in an injury-plagued six-month spell with Shenhua, who said they would not be renewing his contract.
"I don't know why but this is probably one of the darkest moments of my career," Jancker said in the Shanghai Daily.
"I thought I could score a lot of goals when I first came here but it turned out to be very difficult... I should have done much better here in the CSL," he said.
32-year-old Jancker signed a six-month contract with Shenhua in May for about $500,000, but a mid-season thigh injury saw him miss several matches and struggle for form.
Wu Jingui, Shenhua coach, was philosophical about the former Bayern Munich forward's difficult season.
"Jancker basically did not play to his potential. We can't deny his form," Wu told the Shanghai Evening Post.
"Regardless, he is a very good player.
"Had he not been injured, I believe he would have made a bigger mark at Shenhua," added Wu.
Leaving Shanghai this week, Jancker plans to take some time off in Italy before making a decision on his playing future, the paper said.
In his heyday, the strongly built striker was a powerful aerial threat for Germany, and also helped Bayern win the European Cup in 2001 and the German league title four times.