The Hungarian joins for a month after former Bull Dean Beckwith was sent off at the weekend, leaving the Hatters short of defensive bodies.
Manager Gary Brabin told their official site: "It's been well documented that we need more cover in defence and I'm delighted that we've been able to bring in Janos.
"He is a commanding, vocal defender who has vast experience and was the Hereford captain last season. Not only is he a great defender but he's a real threat in the opponent's penalty area."
Kovacs has not started a senior game for the Bulls this season although he has been on the bench on a couple of times.
He has played for the 'reserves' and scored a goal for them recently.