He told Villa's website: "He started as a full-back and played for Liverpool very early on his career, which was a massive achievement.
"As he's got older, he's moved to centre-half. A lot of people say that small centre-halves are likely to struggle in the top flight but he's one of the toughest in the business.
"He's like a rash. When someone takes him on one-on-one, there's no-one better.
"When he wins the ball, he doesn't just get rid of it either. He uses it sensibly.
"A thing that comes across every week too is that it means a lot to him to play for his hometown club. He is very passionate. I think that's the way they are bred in Liverpool. They are football-crazy and kids love playing on the streets so I am sure it was a dream come true for Jamie to play for Liverpool."