The Brazilian fired in an accurate low drive in the 70th minute at Ewood Park to earn Brendan Rodgers’s side an FA Cup semi-final date with Aston Villa at Wembley on April 19.
“Over the two games it was a difficult tie,” said Liverpool manager Rodgers.
“They are strong and well organised. We knew it would be difficult. But my players were brilliant.”
Coutinho seems to enjoy playing cup replays in this part of northwest England, having also scored the winner against Bolton Wanderers in the fourth round.
It was hardly comfortable and it was not pretty, but for Rodgers this victory was more than welcome.
Since Blackburn held out for a goalless draw at Anfield in their first encounter a month ago, Liverpool have seen their hopes of a place in the Premier League’s top four fade, with two defeats in three matches.
By Rodgers’s admission, it had placed extra significance on this competition, which the Reds have not won since 2006, and particularly with the final falling on the day that departing captain Steven Gerrard celebrates his 35th birthday.
“I made the point to the players if we could get through to the semi-final, it could give us a kick on in the league,” Rodgers added.
“We have had two poor performances and been criticised. Tonight we performed very well.”
Rodgers made three changes to the team humbled 4-1 by Arsenal on Saturday, with Dejan Lovren coming in for his first appearance since the match against Rovers at Anfield.
A narrow midfield of Lucas Leiva, Joe Allen and Jordan Henderson helped Liverpool enjoy the majority of possession in the first half, but both teams saw goal-scoring opportunities come and go.
With Rudy Gestede — Liverpool’s tormentor in the first game — only fit enough to come on for the final 26 minutes due to a hamstring strain, Blackburn had Jordan Rhodes as their lone striker for most of the match.
It meant players had to drive forward from midfield to offer support and Craig Conway almost benefited from doing so when an error from Mamadou Sakho nearly let him, only for the Frenchman to slide in and atone.
– Goalkeeper denies goalkeeper –
It was to be Sakho’s last notable involvement before he was replaced by Kolo Toure after appearing to injure his right hamstring.
Rhodes has 17 goals to his name this season, but he headed a glorious chance over the bar from Ben Marshall’s left-wing cross.
Quiet for most of the first half, Coutinho forced a smart save from Simon Eastwood in the Blackburn goal after Henderson’s corner caused problems shortly before the interval.
Liverpool goalkeeper Simon Mignolet had hardly been called into action in the first half, but he had to make two eye-catching saves to thwart Tom Cairney and Marshall early in the second.
The introduction of Gestede lifted the home fans and gave the Rovers players someone other than Rhodes to aim at, but Liverpool weathered the brief storm and took the lead six minutes later.
Coutinho’s poor right-wing corner was volleyed away, but the ball came back to him.
The former Inter Milan playmaker passed to Henderson, darted into space to receive the return ball, and arrowed a low drive in off the far post.
Blackburn manager Gary Bowyer waved his arms, urging his side to lift themselves again, and there was time for one last close call in stoppage time.
Eastwood went up to attack Adam Henley’s long throw-in and managed to slam in a shot on the turn, but Mignolet denied his fellow goalkeeper a famous equaliser to safeguard Liverpool’s place at Wembley.
“Our lads gave it a right good go,” said Bowyer.
“They had a taste for it and it was a great atmosphere in the stadium tonight.”