Richard Avedon’s work has a very recognizable visual style. All of his subjects are (usually) composed against a plain (white) background and are high contrast. His style creates emphasis on the subject’s emotion and personality. Mannion’s portrait of Jay-Z is reflective of Avedon’s style because it uses a plain background and is high contrast and as a result shares the same emotional quality as the portraits done by Avedon. While this may not be correct to say I feel Mannion’s work doesn’t really differentiate himself from Avedon in any way. Besides the more contemporary subjects and more frequent use of color.
Don’t worry about what is correct. It is a valid opinion that Mannion learned a lot from Avedon and beyond that photographed many famous people but cannot be said to have developed his own style.