Valdan said it was too early to tell whether there may be lasting neurological damage following the stroke that afflicted the right side of Peres' brain on Tuesday.
Valdan said it was too early to tell whether there may be lasting neurological damage following the stroke that afflicted the right side of Peres' brain on Tuesday, but the 93-year-old's broader health indicators were good.
"All the parameters are stable - blood pressure, heart rate, blood saturation," Valdan told reporters, adding that that gave him a "certain optimism".
"The chances of survival are pretty good. As for the degree of neurological recovery, nobody can say at this early stage."
Peres, Israel's leading statesman, who served as president and twice as prime minister and won the Nobel Peace Prize in 1994, has had minor health issues over the past year while maintaining a busy public schedule.
He held regular meetings on Tuesday and was in "perfect condition," Valdan said, before suffering headaches. He was taken for an examination at hospital and then suffered what Valdan described as a "massive stroke".
Doctors have sedated him, putting him into a medically induced coma. They briefly revived him from that state overnight and Valdan said Peres was responsive, squeezing his hand and speaking some words. The doctors intend to lower his sedation later on Wednesday to test his responsiveness again.
Peres has been a part of almost every major development in Israel since the country's founding in 1948. In a career spanning nearly 70 years, he has served in a dozen cabinets and was twice a Labour prime minister.
However, even though he ran for office five times between 1977 and 1996, he never won a national election outright.
He shared the Nobel Peace Prize with Israel's former Prime Minister Yitzhak Rabin and late Palestinian leader Yasser Arafat for a 1993 interim peace deal, although that pact never hardened into a lasting treaty.
Rabin was assassinated in 1995 by a far-right Israeli who opposed the peace deal, and it was Peres who took over in office after Rabin's death. At the next election, despite polls showing him far ahead, Peres lost to rightist Likud leader Benjamin Netanyahu by fewer than 30,000 votes.
Valdan, who is married to Peres' daughter, said he did not believe there was an imminent threat to Peres' life, but called it a difficult time for the family.
"We are very close to him," he said. "We are very moved and touched by his condition, but also very moved by the reaction of the Israeli public."