Ivanovic scored the all-important goal as Premier League leaders Chelsea downed hosts Aston Villa 2-1 over the weekend to preserve their seven-point lead.
The 30-year-old Serbia international has shown a knack for scoring important goals in 2014-15 following his winner against Liverpool in the Capital One Cup semi-final last month and that continued at Villa Park as he took his season tally to four in all competitions.
While there have been countless changes at Stamford Bridge since his arrival from Russian outfit Lokomotiv Moscow in 2008 for a deal worth £9.7million, Ivanovic has proved to be essentially an ever-present in the starting line-up and a priceless piece of business by Chelsea.
"Is he one of the club's best signings? I think so," Mourinho said.
"He came immediately after I left and after that he made a fantastic contribution for this club. He is a competitive animal with a big heart.
"What really matters more is what we as a club feel for him and people like John [Terry] and [Gary] Cahill, guys that without them, you cannot do anything.
"A team is an artist like [Eden] Hazard, it is a defender like Branislav. The mix and combination of talent makes a team.
"Every striker in the world would be proud to score that magnificent goal, it is an amazing goal.
"When he scores that goal and it is three or four nil it is a different feeling. He is a great guy, what he is doing for us is unbelievable."
During his seven years in London, Ivanovic has won three FA Cup titles, the Premier League, UEFA Champions League, Europa League and Community Shield.
