The 27-year-old former Arsenal trainee has overcome a viral infection to be in contention and maintains his focus has not been distracted by talk of a transfer.
He told the club's official website: "Obviously you are flattered when you get linked with other clubs, especially the likes of Aston Villa, but I am fully committed to Cardiff and just want to concentrate on doing well for them.
"It's an honour to be linked with them but I am a professional and need to put that to the back of my mind and play well for Cardiff."
Bristol City boss Johnson hopes his side can take the positives from their 4-2 victory over Preston into the match in Wales.
The Robins had struggled for form going into Saturday's game, having won only one of their previous 11 fixtures.
But, after getting their push for the play-offs back on track, Johnson is hopeful they can transfer their new-found belief into the cup.
"A result like that definitely helps," said Johnson, who is likely to be without striker Danny Haynes because of a hamstring injury.
The sides fought out a 1-1 draw in the rearranged first tie last Tuesday, with Gavin Williams' 92nd-minute equaliser earning City a replay.
The winners face a fourth-round home tie against Championship rivals Leicester.
Cardiff v Bristol City To Score in 90mins: Bothroyd, J. 6/4