Long set up Jelavic for the Tigers' second goal at the Cardiff City Stadium on Saturday before the Croatian found the net for a second time later on.
"They've only been at the club for three or four weeks and already Jelavic has three and Longy has a couple which helps overall," Bruce is quoted as saying by Sky Sports News.
"It's difficult for a team that have just come out of the Championship and to put in the work ethic that we require - both of them have it in abundance. As you've seen today, they're both very, very good players."
Hull return to action on Monday when they host Brighton & Hove Albion for a replay of their FA Cup fifth-round tie.