Dennis Wise told Sky Sports HD that Branislav Ivanovic will have "nightmares" about the mistakes that helped Swansea seize control of their Capital One Cup semi-final with Chelsea.
The Blues defender's poor touch on the edge of his penalty area allowed Swans forward Michu to open the scoring after 39 minutes, and a blind back-pass in stoppage time was pounced upon by Danny Graham; the substitute rushing in to net a second for the Welshmen.
Swansea only had four shots, compared to their hosts' 20, but will head to the Liberty Stadium for the return leg on January 23 in pole position to claim a spot in the Wembley showpiece.
And former Chelsea skipper Wise says Michael Laudrup's charges can thank their defensive resolve, and not just Ivanovic's errors, for their two-goal advantage.
"Ivanovic, who is usually very solid, made two poor mistakes, which he will be having nightmares about," said Wise, who won the League Cup with Chelsea in 1998. "His overall performance was great, but those two moments where he lacked concentration have really cost his team.
"I think Chelsea played extremely well and had a lot of possession and chances, so they will be disappointed to have lost the game, but the fact Demba Ba's goal towards the end was ruled out for offside, when it probably shouldn't have been, summed up their luck.
"You have to give Swansea credit, though, because they defended fantastically as a team; they stuck at it and worked really hard for each other and have got a fantastic result to take to their place."
Wales manager Chris Coleman highlighted the performance of his countryman, Swansea centre-back Ashley Williams, praising the former Stockport stopper's belligerence and composure at the heart of the back four.
But the ex-Fulham chief warned the Swans that they have only completed half the job, and expects Chelsea - who have now lost successive home games, following the Premier League reverse to QPR - to come out firing in South Wales.
"Williams showed great anticipation and wherever the ball was going to arrive he was there like a magnet," said Coleman.
"He put his body in front of things; was very aggressive, both aerially and on the ground; and read situations very well.
"Swansea are a passing team but he is experienced enough to know when not to pass it and is not afraid to clear his lines. He is a great leader, a tremendous centre-back and helped Swansea get an unbelievable result.
"If Fernando Torres had Williams' anticipation, Chelsea may have scored one or two goals, but with the players Rafa Benitez has at his disposal, his team are not out of it, and if Chelsea get the first goal at Swansea it is back on."
Catch the second leg of Swansea and Chelsea's Capital One Cup semi-final, from 7pm on Sky Sports 1 HD and Sky 3D on Wednesday, January 23.