The Zebrette lost 1-0 on Saturday, with Samu Chukwueze scoring for Rossoneri, who also lost Tijjani Reijnder to a red card on 29 minutes.
Runjaic admitted afterwards they should've done more with the extra man advantage.
He said, "Congratulations to Milan for this victory. I'm not happy with the result, I'm not happy with the first 30 minutes. Then we played with much more confidence. I'm not happy with the final result, but I think we tried, especially in the second half, to play football and it was a one-sided match.
"Then we weren't lucky with the referee's decisions. I don't want to comment on them, it's over now.
"It's not easy to play against such low (block) teams, we are not Manchester City... It's a shame not to go home with a draw."
Asked about Iker Bravo, Runjaic said: "He's in a good moment and I gave him the chance to play. Unfortunately I didn't have Thauvin available. Iker tried, he's very young and at the beginning of his career. He has to work hard.
"Today we could have played with 7 forwards, but if the timing in positioning ourselves in the area had been the same we wouldn't have scored anyway. I expected something more from Milan even with 10 men, given the quality of the team, instead they contained the game. So, even more so, it means that Udinese played a good game if Milan chose to only contain the game."