The former Aston Villa centre-forward struck from the spot in the 3-0 win on Monday night.
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Afterwards, Runjaic said: "The win was very important.
"We played a serious match, and it wasn't easy after such a difficult period. I saw energy and passion, fighting until the last minute."
On Davis' performance, Runjaic added: "A very important player, he's performing at a different level this season when he's healthy.
"I'm happy he's back, even if he wasn't even 100% tonight. It was a great Monday night."