The new-look strike force both found the target and posed a persistent threat as the Tigers downed Sunderland at the Stadium of Light.
Long netted for the second time since moving to East Yorkshire, while Jelavic opened his tally with an opportunistic finish on the hour mark.
Victory over the Black Cats ended a five-match winless run for Hull, but despite recent trials, Harper believes signing two proven strikers displays the club's ambition.
"Buying those two has been a huge, huge statement from the club and for both to get a goal is absolutely fantastic," he told the Daily Star.
"Hull does feel like a club on the up even though we have had a bad run.
"We always knew the chances were that we would have one at some stage.
"The big thing was that ours came at a time when the board could do something about it and, fair play to them, they have done that."
Bruce, formerly manager of Sunderland, is synonymous with the 'teams are only as good as their strikers' ideology and the Hull boss is impressed with his recent acquisitions.
"Having Long and Jelavic can only help us," he said in the wake Saturday's win.
"When we had four strikers at Sunderland, we were always comfortable but we lost all of them and then it was a struggle.
"For a newly-promoted side, it's really good having forwards of that quality. I was shocked, though, because I thought there would be more competition for Long but the kid wanted to come here."