"I said that if no solution with guarantees for treatment and legal guarantees for all opposition prisoners as well as any people were found, then the attitude to such a country would be certainly critical, and certainly one or another level of isolation would be guaranteed," Dalia Grybauskaite said after talks with the Ukrainian leader in Kyiv.
Grybauskaite also said Ukraine has much "homework" to do.
"The homework that needs to be done, and reforms -- they are not easy -- it's also not that easy to explain them inside the country. Certainly we -- Lithuania and other countries -- have also been going the way of guarantees and human rights, and reforms of our judicial system, for it not to be a repressive structure but to protect people and their rights," Grybauskaite said.
Earlier, Grybauskaite met with Tymoshenko at a hospital in Kharkiv -- the first foreign leader to have been granted access to her since her conviction in October 2011.
The former prime minister is being treated for back pain and is recovering from a hunger strike that she started in April after alleging that prison guards beat her.
"[Yulia Tymoshenko] is in good condition, considering that she's now recovering from a 20-day hunger strike," Grybauskaite told journalists. "Her mood is positive and optimistic. She did not complain about anything. On the contrary, she is glad that doctors are finally taking proper care of her."
Hundreds of people gathered in Kyiv on May 12 to protest the Yanukovych government and to express support for Tymoshenko.
Demonstrators called for Tymoshenko's release as her daughter, Yevhenia, read a letter from Tymoshenko calling for new mechanisms to empower ordinary citizens to start impeachment proceedings against state officials.
Tymoshenko is serving a seven-year sentence for her October 2011 conviction for abuse of office -- a charge she denies and which she says was politically motivated.
Tymoshenko's plight has triggered an outcry in the West and led to boycott threats by European Union politicians of next month's European soccer championship, which Ukraine is co-hosting.
Earlier this week, Yanukovych canceled a summit with Central and Eastern European leaders on the Black Sea after several of the key participants said they were staying away.
Grybauskaite said Tymoshenko had expressed concern over Ukraine's European Union prospects.
"Ukraine's European prospects and European integration are very important to her," Grybauskaite said, "and she is calling on everybody -- the government and the people -- to remember that reforms are necessary and that Ukraine must take the path of European integration and should not be isolating itself."