Seibutis was drafted in the second round with the 50th overall pick of the 2007 NBA draft by the Dallas Mavericks. Following the draft selection, he played with the Mavericks' NBA Summer League team in Las Vegas, averaging 6 points, 1 rebound, 1.6 assists, while shooting over 47% in 13 minutes per game over 5 games. He played again in 2008, averaging only 2 points in 10.9 minutes per game over 5 games. He was ultimately dropped from the 2008 roster because of his underperforming.