BUENOS AIRES, June 25 -- Argentina forward Lionel Messi turned 31 on Sunday as his embattled national team prepared for its next match at the 2018 FIFA World Cup. "Happy Birthday captain Leo Messi," the team posted to its Twitter account, along with three photos of the talisman blowing out candles on three birthday cakes. It seemed like a low-key affair well-suited to the guarded mood of the team, whose match against Nigeria on Tuesday will be decisive. To advance to the knockout stage, Argentina have to beat the African team and hope Iceland don't defeat Croatia in the other remaining Group D game. There have been rumors of strained relations between the players and the coaching staff, despite assertions to the contrary by midfielder Javier Mascherano, who plays his club football for Chinese Super League outfit Hebei China Fortune. At a press conference on Sunday, Mascherano said the relationship with head coach Jorge Sampaoli "is completely normal." Sampaoli has come under fire since his side lost their second group game of the tournament to Croatia 3-0, following a lackluster first game against Iceland in which they drew 1-1. According to Argentinian media, the coach and players have been at odds over Sampaoli's constant changes in strategy and personnel. The Albiceleste, who finished runners-up in the 2014 World Cup, are facing the possibility of being eliminated in the group stage, as they were at the 2002 tournament under the stewardship of Marcelo Bielsa.