The striker was again on the scoresheet in victory over Reading.
"He's just flourishing under the new manager," said Lallana. "I've always said he's going to be a big player and he's proving it now. He's unbelievable.
"There's no limit to where he can go. He's got all the attributes, physical and technical.
"The main thing is he's a top lad, got a great attitude, wants to do well for the team and it's a pleasure to have him at Southampton."