The 23-year-old became the first Reds player since Ray Kennedy in 1974 to score in his first three matches for the club with a close-range effort in the 5-0 demolition of Norwich.
"He's done very well. He gives us a bit more pace and strength up there," said the veteran centre-back. "He has a presence about him, he looks a really good player and has linked up well with Luis (Suarez) playing the slightly deeper role."
He continued: "He has got three in three games so he's made a great start and he's lived up to the expectation so far.
"But it is not just about goals - that's an important thing for a striker - but he's got great movement and pace.
"You can't expect a striker to score every week but he gives us that pace to get in behind.
"When he came in it gives everyone a lift and raises standards all around.
"The other strikers have to up their game to try to displace him and get in the team.
"It is early days for him but he has started well and it looks like a good decision to bring him in."