Gestede scored his second goal of the season on his first start of the campaign in the eighth minute, with a close-range finish following good work by Craig Noone. That was enough for Cardiff to record a 12th home win in 13 matches and keep hold of their advantage over nearest rivals Hull.
"Rudy is an outstanding professional. He came in, took his chance and will play a major part in what we are trying to achieve this year. He is a huge option for us," Kerslake said.
"We didn't have the strength of depth last year. Now we've got players like Heidar (Helguson) and Don (Cowie) coming off the bench.
"We couldn't do that a year ago. Certainly our squad is one of our big strengths now."
David Marshall produced neat saves to stop Darius Henderson, Danny Shittu and James Henry and keep Kenny Jackett's side out. But Kerslake rejected the claim that the Bluebirds were fortunate to claim another three points towards their promotion target.
Kerslake added: "Not at all. We started exceptionally well. We had a really high tempo in the first 25 minutes, got at them and scored the early goal."
This was Millwall's first game without Chris Wood, who scored 11 goals in 19 games during his loan spell with the Lions. The West Bromwich Albion striker is expected to join Leicester in a £1.25million move when the transfer window reopens on Tuesday.
But the south Londoners created plenty of chances in his absence as the opening period wore on.
Millwall chief Jackett said: "It was a good performance and the positive was the amount of chances we created. You need to finish those off. But we didn't go down easy at all. We have to get over Chris leaving. The people above me tried very hard to keep him. We are disappointed, but they could not do anymore in our remit."