Former Chelsea goalkeeper Carlo Cudicini lauded Thibaut Courtois for his seamless transition to the Premier League.
Jose Mourinho's side are on track to win the Premier League title, sitting 10 points clear of Manchester City and Arsenal.
Cudicini, who was at Chelsea from 2000-09, said the signings made by Mourinho were crucial to the club's success.
Diego Costa, Cesc Fabregas and Courtois – who returned from his loan spell at Atletico Madrid – have all been key players this season.
"I think the success started last year when the club signed those players, important players that straight away walk in the team and had a great impact and has been the start of a great season," Cudicini said.
"Fabregas, Costa, Courtois are the spine of the team, and have had an amazing impact on the team.
"Fabregas has 16 assists which is unbelievable, Costa scored important goals from the start which is what the team is looking for and Courtois is quite young, but he's such a talent and so much experience for someone of his age.
"He [Courtois] gives security at the back, he's straight into the team. Coming from a different league isn't easy, especially as a goalie, but you can't tell the difference.
"[They are] important players for the club and that shows you how right they were when they picked those players."
Another key this season has been Eden Hazard, who was named the PFA Player of the Year.
Cudicini backed the decision to reward the Belgium international for his season, despite Manchester United goalkeeper David de Gea's fine campaign.
"I feel he [De Gea] has had a great season but Eden Hazard deserves the award. He's had a fantastic season," he said.
"He's scored many important goals for the team, assists, he's been a fantastic and important player throughout the season and so I believe he fully deserves it."
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