Doubts remain over the £125m signing from Bayer Leverkusen after a slow start to his time with Liverpool.
But Mellor told Sky Sports on Sunday night: "I was made up when he signed for Liverpool.
"And I was thinking: 'Phew, that's brave!'
"Bayern Munich wanted him and it would have been an easy option to stay in the Bundesliga and sign for Bayern Munich. So he's been really brave to come to the Premier League.
"We know it is the best league in the world with the physicality, the intensity, and it will take time to adjust at Liverpool. I've seen this lad this season and forget what social media says, this lad is an unbelievable player.
"He hasn't started all the games, (some) people are asking for him to be taken out and he has been at times, he didn't start the Merseyside derby against Everton, against Manchester United, against Chelsea. So he has been given a bit of time to try and adjust.
"But there is a player there. There is an unbelievable talent of a player who can find pockets of spaces against really compact defences and make things happen. And people will say he hasn't got an assist and I can't argue with that but he does create chances.
"He creates chances and Liverpool have been a bit wasteful with some of those he has created. I don't know where his position is long term. He played against Arsenal and Atletico Madrid this season, which were fantastic results for Liverpool, as the No.10.
"He was that No.10 in there and he got into some great positions, outstanding. Against Real Madrid, it was the best performance from Liverpool this season and he was out on the left, drifting inside. Against Frankfurt, he played in a 4-4-2 on the right-hand side, he is capable of playing in different positions."