The Reds missed out on the signing of Virgil van Dijk last summer to leave Jurgen Klopp with Dejan Lovren and Joel Matip as his first-choice centre-halves.
The pair struggled in the opening months of the season as Liverpool FC conceded nine goals in heavy losses to Tottenham Hotspur and Manchester City.
Liverpool FC signed Van Dijk in a world record fee for a defender following his £75m switch to the Anfield outfit from Southampton in the January transfer window.
Klopp reinstated Loris Karius as his number one goalkeeper ahead of Simon Mignolet, while Andy Robertson moved ahead of Alberto Moreno in the pecking order.
Former Manchester United captain Keane has been impressed with Liverpool FC’s defensive improvement over the past few months.
“[The defence] always the big concern for me,” Keane told ITV Sport.
“Earlier in the season some of the performances away at Man City, away at Spurs, you just felt they were going to concede, especially against the biggest teams.
“But they have improved that over the last few months. With the signing [of Virgil van Dijk] they have a settled back four, Andy Robertson has come in and done really well.”
The Merseyside outfit were 3-0 winners against Manchester City in the first leg of their Champions League quarter-final earlier this month.
The Reds managed to recover from a 1-0 deficit to win 2-1 against the Premier League leaders in the second leg at the Etihad Stadium.
Liverpool FC are in third place in the Premier League table and on course to finish in the top four in successive seasons.