"I haven't seen that in a long time, even at training. Some signs were disappointing and we will have to change it pretty quickly."
Asked about their missed tackle count, Robinson said: "I put it down to the Broncos playing really well and us being unacceptable the way we went about our tackling."
Before the match, the Roosters had the league's fourth best defensive unit after conceding less than 16 points a game in the first five rounds.
Defence is clearly a concern but Robinson also had a problem with their attack.
The Roosters are the only team not to have won a second half this season, losing their last 15 games while trailing at halftime.
"I thought we handled them poorly early, they got the jump on us," Robinson said of his side's fourth consecutive loss in Brisbane.
"It was an open game and in the second half we couldn't handle it.
"We couldn't offer the same attack that we offered in the first half to get back into the game."
Meanwhile, the Roosters have appointed former Manly boss Joe Kelly as chief executive.
Kelly, a long-time South Sydney chief financial officer, also has experience with Chelsea in football's English Premier League and left Manly last year.
"Joe has over two decades' experience in sports administration both in Australia and overseas," Roosters chairman Nick Politis said.
Kelly presided over a difficult time at the Sea Eagles, brought in after the Penn family bought a controlling stake of the board, ending years of fractures. He was at the helm when Geoff Toovey was sacked as coach and Trent Barrett took over.
