The Spain players met up with their new manager for the first time on Monday to prepare for the international friendly in Belgium on Sept. 1 and their opening World Cup qualifier at home to Liechtenstein on Sept. 5.
Casillas was the most high-profile absence from Lopetegui's squad announcement since succeeding Vicente del Bosque, although Pedro Rodriguez and Cesc Fabregas, who won the 2010 World Cup and Euro 2012, were also excluded.
"We were all surprised by the absence of Casillas because he is a historic player, not just in Spanish football but on a global level," Ramos told reporters on Monday at Spain's Las Rozas training base.
"It's odd to come here and not see Casillas but it's a decision he took with the new coach. I can't speak for him, all I can do is respect the decision they have taken."
Ramos met with Lopetegui last week.
"I didn't know him but I've got a good first impression and we're going to work together to achieve the objectives we have ahead," said Ramos, vice-captain under Del Bosque and now the team's skipper following Casillas's exclusion.
"Apart from the absence of Iker, which is surprising, the group hasn't changed a lot and the captains and coach want to recover our winning spirit," Ramos added.
"We have to be calm and patient with Lopetegui and let him get to work. We all have to row in the same direction."
Casillas made 167 appearances for Spain and captained them to the 2010 World Cup win and successive European Championship titles in 2008 and 2012.
The Porto goalkeeper was not picked for any of Spain's games at Euro 2016, Del Bosque choosing David de Gea as his number one, and the coach revealed last month that the captain had reacted angrily to his demotion.
"With his team mates he was OK, but his behaviour with the coaching staff wasn't good. His anger was directed at us,” Del Bosque said.
Del Bosque also revealed Casillas was the only player he did not write a letter of gratitude to after resigning as coach following Spain's elimination by Italy in the last 16 of Euro 2016.
(Editing by Ed Osmond)
