The goalkeeper's mistake allowed the Championship promotion chasers to equalise.
Rowett's men were 1-0 up thanks to Stephen Gleeson's goal just after the half-hour mark but went into the break level with the automatic-promotion contenders after the 29-year-old spilled Gaston Ramirez's free-kick straight into the path of Jordan Rhodes.
But the Blues manager has defended the stopper by pointing out that it was down to him that the scoreline was not worse for them by that point.
"It could have easily affected him but it didn't. It was a shame for him because he made two fantastic saves before that to keep it at 0-0," he told the Mail.
"There is a patch in front of the goal that has been relaid and it might have just taken a little bit of an extra bounce from it.
"He will learn from it, he has waited patiently for his opportunity, he has done well when he has had his chance, yes it was a mistake but that's part of the process of getting better."
With Legzdins being Tomasz Kuszczak's understudy for much of the campaign, Rowett will surely be more concerned about the form that Birmingham have shown in recent weeks than by the goalkeeper's blunder.
They have won just one of their last 11 games in the league, which has seen them fall from being genuine play-off contenders to being in danger of missing out on finishing inside the division's top 10 altogether.
And with a trip to a Cardiff side that may go into the final day still with hope of finishing inside the top six, Rowett may not be too confident that there will be a positive conclusion to a promising season.