If you reads the laws, it's a foul.
TAA challenged for the ball and in doing so tripped DCL, and by leaving his feet and sliding in front of him was careless.
I do think its a "soft" pen, but they still count as fouls, he was trying to block the shot and was unlucky that DCL tripped over him. But it's still a foul.
As for Carragher asking what's he supposed to do, stay on his feet if you can't make that challenge without tripping your opponent, it's quite simple.
Although to be honest, i think if Alisson has held the shot it wouldn't have been given, you see strikers took out after getting the shot away all the time but the foul very rarely gets given, but as he spilt it and DCL might have got to it then it got given.
I don't think it was a red because it was an attempt for the ball, all be it a poor one rather than a shirt pull or similar, and it's hard to say if DCL would have got there before the keeper.