Long had only arrived at the KC Stadium in January, but signed a four-year deal on the south coast after being snapped up for a reported £12 million.
And Bruce conceded that, while the club are delighted with the money they received for the Republic of Ireland international, they can waste no time in sourcing a replacement, with the Premier League season set to get under way on Saturday.
"It's like anything in football. Even if you're Cristiano Ronaldo or Luis Suarez, you have a price," he said. "We thought the offer that came in was too good to turn down.
"Now we've got to find a replacement, but we will invest the money into the squad and make the squad bigger and better.
"We obviously hope we can find a replacement who can be the equivalent of Shane Long.
"Yes, we've got our irons in the fire to make an instant replacement, but if I'm being honest, I would like at least another three players coming through the doors.
"With the money that we've got for Shane, that allows me to do that.
"We'll be active over the next few weeks, and it's all about making sure the squad gets bigger and better."
Bruce, who confessed his shock at losing Long, rubbished reports linking him with moves for Manchester United's Danny Welbeck, Watford's Troy Deeney and Hamburg's Rafael van der Vaart, but did reveal that he had a few targets in mind.
"It came so quickly with Shane," he added. "I never envisaged this time last week that Shane Long would not be with us.
"However, as a manager you always have to have something up your sleeve.
"There's always people who are on the radar.
"Sometimes you can't get them, sometimes it's not possible to bring them in, but make no mistake, with the money we've got we'll invest it back into the squad."
Hull get their league campaign under way at QPR on Saturday.
