Perhaps thinking of their Champions League tie in Rome in midweek the Barca players seemed to take their foot off the gas and Nabil El Zhar gave Leganes hope in the 68th minute.
European media outlet Don Balon say Messi - who is a hugely influential figure at the club - has told Barca bigwigs to let Alcacer leave.
Sergi Roberto at Left-Back? Summer signing Ousmane Dembele has offered an extra outlet of pace after recovering from injury, while the arrival of Philippe Coutinho in January added another layer of class to a group of players already sizzling with quality.
Messi clearly proved his fitness to Valverde and then went on to show he is still in peak form. And this was yet another display of the Argentinean's brilliance, with Barcelona once again dependent on their number 10. As with three minutes remaining, the 30-year-old bagged his hat-trick after eventually receiving possession around six yards from the Leganes goal before lifting his attempt past the encroaching Cuellar to secure a potentially title-defining three points. Barcelona is now at the top of the La Liga table and should be crowned champions in a few weeks.More news: API shows fall in United States crude-oil inventories
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Second half staminaBarcelona have fallen behind eight times in the league this season but fought back to draw against Atletico Madrid, Valencia, Espanyol and Sevilla, while they rallied from a losing position to beat Real Sociedad, Getafe, Girona and Alaves. They are seven games short of becoming the first team to complete a 38-game Liga season unbeaten. "It has been quite hard, because of the fact it's an old record that hasn't been broken for a long time".
Barcelona's defence has been their great strength, but there's no doubting the team switched off after going two goals up.
Ernesto Valverde picked a strong side for the fixture despite facing a Champions League quarter-final second leg against Roma next week, with only Jordi Alba, Andres Iniesta and Samuel Umtiti rested. The Catalan giants will be favourites to progress, as they hold a commanding 4-1 lead from the first leg.