Croatia lost 4-2 in the 2018 World Cup final to France
Croatia reached the final in Russia in 2018, beating England in the semi-finals, before losing 4-2 to France.
"What we went through in 2018 was an unforgettable experience, indelible in our minds," the Croatia captain said.
"We will speak about it and be having recollections for the rest of our lives, but we need to put it aside."
Modric, now 37, collected the Golden Ball award in Russia for being the tournament's best player and added: "Many players at the Russia World Cup are not here, new players have come, quality young players with fresh energy.
"This is a new tournament and we have to look at it that way.
"We're expected to play our hearts out, give our best and leave the last atom of strength on the pitch.
"If this means we get good results, then great - if not, we have to keep our heads high knowing we gave our best."
The match at the Al Bayt Stadium in Al Khor is the first game on Wednesday (10:00 GMT) and the opening one in a Group F which also contains a Belgium side ranked second in the world and a Canada team playing in their first World Cup since 1986.
Croatia are 12th in the rankings, 10 places above Morocco, a nation that has played at five previous tournaments but only made it out of the group phase once, in 1986.
Walid Regragui replaced Vahid Halilhodzic as Morocco head coach just three months ago and one of his first tasks was to recall Chelsea midfielder Hakim Ziyech to the national side.
"Our players will need the best of Hakim Ziyech," Regragui said. "When Hakim has this confidence, he is one of the top players in the world.
"We need him with his quality and he will give everything.
"It's going to be a very difficult match, we will adapt and play our hearts out. We will show great motivation and respect for the opponents.
"Of course, Croatia are favourites in this game - they played in the final of the last World Cup and have the Golden Ball player in Luka Modric.
"What's important is we play Morocco's football with a positive attitude, give everything and afterwards we have no regrets."
Morocco defender Nayef Aguerd has recovered from the ankle injury he picked up in pre-season, despite only making one West Ham appearance in the Premier League this season - as a substitute in the 2-0 loss against Leicester on 12 November.
Croatia have no fresh injury concerns, with Chelsea midfielder Mateo Kovacic available despite struggling with a knee injury recently.
-- Courtesy of BBC Sport