Herrera has become a regular in Jose Mourinho's Manchester United team, and recently won his first international cap for Spain.
Manchester United's Ander Herrera reacts after fouling Tottenham's Christian Eriksen
He was impressive for United at the weekend in their 1-0 win over Tottenham, creating five chances, more than any of his teammates.
Redknapp said: "Mkhitaryan's goal was set up by Ander Herrera, who has been United's unsung hero.
"He is by far the most improved player in the league this season."
Manchester United's Ander Herrera in action with FC Zorya Luhansk's Zeljko Ljubenovic
Herrera, 27, is still looking for his first Premier League goal of the season, although has one to his name in the EFL Cup.
He is into his third season at Manchester United, after a couple of mixed seasons under Louis van Gaal.
A reduced role for the ageing Michael Carrick has seen Herrera get more opportunities, while Marouane Fellaini and Morgan Schneiderlin in particular have fallen out of favour.
Herrera's contribution for United has largely been overlooked in recent weeks due to their inability to close out games, but after their win over Tottenham at the weekend, he is finally getting some much deserved credit.