Lorenzo Lucca and Florian Thauvin (2) struck twice for Udinese's triumph.
Runjaic later said: "We definitely had more courage in possession in the second half. In the first we didn't do well, but not that badly either. We conceded goals on two mistakes and wasted important scoring opportunities.
"At half-time we told ourselves that it wasn't impossible and we showed courage, it went well.
"We are happy with this moment but it's only the beginning. It's important at this stage to get as many points as possible. It was a deserved victory because we made a great comeback in the second half and the first half wasn't bad either. We mustn't look at the standings, we must remain cool and lucid and work on the details."
Asked about the differences between the Polish and Italian leagues, he added: "I don't like to make comparisons, every championship has its difficulties, in Polish there is more physicality, here the level is higher and we work with more quality and tactics. The Italian championship is among the best five."