"This is like living a dream", Coutinho told the club's website.
So far this season in 17 league appearance, Messi has scored 14 goals and created 45 chances with six assists.
Leo Messi played his 400th match in La Liga and celebrated the milestone with a goal.
Coutinho will be officially presented Monday after a transfer deal that could reach 160 million euros ($192 million).
The referendum had direct ramifications for Barcelona who, on the day that it was held, chose to play their La Liga game against Las Palmas behind closed doors in response to police violence against would-be voters.
"We hope he will give us a lot".
Not that Barcelona needs it in the league.
As Ousmane Dembele made his first league start in more than three months, due to hamstrong injury, Barcelona put up a routine win but in an underwhelming manner.
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.
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.
The second goal arrived seven minutes before the interval, as Suarez controlled Sergi Roberto's fine ball into the box before firing home.
Furthermore, they are without key personnel in Chema, Roger Marti, Ivan Lopez, Jose Campana, Enis Bardhi and top scorer Jose Luis Morales for the trip to Catalonia.
Barcelona reserve-team striker Jose Arnaiz debuted in the league as a late substitute.
Celta outplayed Madrid in the second half and its defenders left unmarked striker Maxi Gomez to head in a pass by Wass for an equalizer in the 82nd.
"I have to find out what the problem is", coach Zinedine Zidane said. He had done so during negotiations for his most recent renewal.