A new-look Atlanta United launch the defence of their Major League Soccer crown on Sunday as the new season kicks off with FC Cincinnati looking to become the latest expansion side to make a mark on the competition.

Less than three months after Atlanta crowned their meteoric rise with victory in the MLS Cup over Portland, the champions start their campaign against Wayne Rooney and D.C. United.

The Eastern Conference clash at Audi Field will be a difficult first test for Atlanta, who have had to absorb the departure of title-winning coach Gerardo "Tata" Martino and star player Miguel Almiron in the close season.

Martino has been replaced by former Dutch international Frank de Boer, returning to management after a disastrous spell at Premier League side Crystal Palace, where he was sacked after five games and 77 days in 2017.

De Boer has been entrusted with rebuilding a team that saw Paraguay international Almiron leave for Newcastle United in January.

Atlanta had already planned for Almiron's departure by recruiting Argentine midfielder Gonzalo "Pity" Martinez from River Plate.

Martinez, who helped River win last season's violence-scarred Copa Libertadores, hopes to use his spell in the MLS as a stepping stone to Europe, the latest South American player aiming to follow Almiron's route.

"I think the level in this league is really growing, and this is a club who's doing things well," Martinez said.

"They're coming off a championship, so those are the things that motivated me to come. ... (The chance to move on to Europe) was a factor, too. You see players leaving MLS for Europe, so that played into my decision."

On Sunday, Martinez will likely line up against D.C. star Rooney, the former Manchester United and England striker who transformed the club's season last term.

Rooney, who will captain D.C. United this season, said Almiron's departure reflected well on the growing maturity of MLS, which enters its 24th season in rude health.

"I think that's great for this league to have players going over, to England, to Spain wherever they go and to perform," Rooney said. "It will give fans around the world and in Europe maybe a different opinion on MLS."

Rooney, who scored 12 goals in 21 games for United last season, admits he had been impressed by standards in MLS since his arrival last July.

"I think the standard of play maybe took me by surprise a little bit with the amount of quality there is in the league," he said.

One club hoping to emulate Atlanta's template for success are FC Cincinnati, who become the league's 24th team after switching from the USL professional league.

It remains to be seen however whether Cincinnati have the resources to challenge in the Eastern Conference. The club only began training in January.

Cincinnati's South African manager Alan Koch has cautioned fans against expecting too much, too soon. "I want everything for this club," Koch said. "But we're dealing with a very, very short timeframe. We all have to remain patient through that process."

Cincinnati start life in the MLS with a challenging trip to perennial Western Conference contenders Seattle on Saturday.

The Los Angeles Galaxy meanwhile enter a new era under the guidance of new coach Guillermo Barros Schelotto, who arrives in California after taking Boca Juniors to the Copa Libertadores final last year.

After dominating the league in the early part of the decade, with three titles between 2011 and 2014, the Galaxy have nosedived in recent seasons, failing to make the playoffs in both 2017 and 2018.

Swedish striker Zlatan Ibrahimovic, who led the team to the brink of the postseason last year, is confident that better times lie ahead.

"I think you will see things you didn't see the first year from Galaxy," Ibrahimovic said.

"We are on a different page now so, but that I don't see as a surprise. That should be normal for us."