Dowie believes that it was due to the former Arsenal prospect that Forest failed to pick up a point against Wolverhampton Wanderers at the City Ground on Friday.
The 24-year-old made a needless sliding tackle on Bakary Sako in the 72nd minute inside the Reds' penalty box.
Sako himself took the spot kick and successfully converted it, making it 2-0 to Wolves in the process.
Dexter Blackstock halved the deficit in injury time, leading Dowie to suggest that Dougie Freedman’s side would have picked up a point had it not been for Lansbury’s mistake.
“I think Dougie Freedman will be having a word with his captan Henri Lansbury, who made a challenge which is just so ridiculous and it ended in a penalty,” the 50-year-old told Sky Sports.
“In the end, that cost them a point… If he didn’t make that silly challenge, then they may have got a point out of that game.”
The defeat to Wolves has severely dented Forest’s chances of making the Championship playoffs this season.
The Reds are currently ninth in the league table with 57 points from 40 matches, 11 points behind sixth-placed Derby County.