The Spaniard was a huge success at the Blues, helping them to several domestic successes.
Abraham got his first two goals for the club against Norwich City at the weekend.
Speaking about Abraham, Redknapp told the Daily Mail: "Let's give Frank Lampard credit for the way he secured his first Premier League win as a manager at Norwich.
"There was pressure on his side to win but Lampard had the courage to start with Tammy Abraham up front and his bravery was rewarded as the 21-year-old got two goals.
"Abraham is not the finished article. I think he needs to be nastier and a master of the dark arts like Diego Costa.
"Then maybe the No 9 shirt will be his for the foreseeable future."