Such an emphatic victory comes as Barcelona's attacking threat is preparing for an additional injection of creativity with the 25-year-old Coutinho.
Messi, 30, made his debut for Barca in 2004 and has since become one of the most revered footballers of all time, leading the club to four Champions League titles.
Ernesto Valverde's side refused to let their 1-1 Copa del Rey draw at Celta Vigo on Thursday spoil their trip to the Hospital de Sant Pau in Catalonia.
He marked his 400th game in La Liga by opening the scoring in the 12th minute, when he volleyed Jordi Alba's cushioned header into the net.
Barcelona are in relentless form at the moment and the scary thing is they're only going to get better now that Coutinho has joined from Liverpool for £142 million.
Barcelona sign central defender Yerry Mina from Palmeiras
He helped the team win Brazil's notorious competitive Campeonato Brasileiro for the first time since 1994 in his debut season. The Colombia global will sign a 5-1/2 year contract with the Spanish side.
Doug Jones Predicts 'Tug-of-War' for His Vote in the Senate
Biden campaigned for Jones in Alabama and recorded a robocall for his fellow Democrat in the final days of the campaign. Democrat Doug Jones of Alabama is greeted by a supporter after his victory December 12.
NFL forecast: Odds not in favor of Raiders' Del Rio
According to Mehta, the Jets would not stop Morton from reuniting with the head coach after just one season on Todd Bowles' staff. Del Rio became head coach in January 2015 and won 25 games and lost 23 in three seasons, including a 6-10 season this year.
Ousmane Dembele and Luis Suarez both came close before the Uruguayan forward battered home the second in the 38th minute. The striker controlled the ball with his left foot before sweeping home with his right for his 11th goal of the season and his sixth in a five-round scoring streak.
However, Messi insists that he will continue with Barcelona regardless of Spain's political climate.
Barcelona reserve-team striker Jose Arnaiz debuted in the league as a late substitute.
But they weren't enough for the win amid an uninspiring performance by Cristiano Ronaldo, and poor defending by Marcelo and the rest of Madrid's backline.
The three points cancelled out second-placed Atletico Madrid's win over Getafe on Saturday to restore a nine-point advantage. He had done so during negotiations for his most recent renewal.