Boro made an offer of around £4m for the Belgian international on deadline day.
However, Genk sporting director Dirk Degraen has since been quoted as saying that the bid was "timid".
"We have been respectful of their club," Mowbray told the Northern Echo. "He would have done well for us and played most weeks. The word timid, which they suggested, was a bit disrespectful."
Mowbray added that he could yet go back in for the player during the January transfer window.