An own goal from Nikola Milenković put Brentford ahead on the half-hour mark, with Igor Thiago adding a second in the 76th minute after Forest had struggled to turn their attacking opportunities into goals.
Glasner was frustrated by the familiar problems that undermined his side's performance, insisting that positive statistics and promising spells were no longer sufficient.
“It's the same pattern; too many mistakes in the defence and we can't convert our chances.
“If these things always happen the same, then we need to change a few things. This is what we have to do and what I will definitely do. Then let's see.
“We can't always say that the stats look fine because we don't score goals, we don't convert the chances we have and we concede too many easy goals.
“We can't keep saying it's OK because we have lost four of our last five matches. It's not OK and not good enough. This is what we have to change.
“There are enough positive signs in that we could have taken wins from all the games. But it's all about the finer details.”
While Nottingham Forest were left searching for answers, Brentford head coach Keith Andrews was delighted with the response from his players after a demanding week of fixtures.
The Bees had travelled to Hull City and Sunderland before returning to west London, but Andrews praised their ability to adapt against an opponent he considered particularly difficult to play.
“I'm very proud of the performance,” Andrews said. “We showed different ways of playing today.”
Andrews acknowledged the scheduling challenges but refused to use them as an excuse, highlighting the importance of maintaining concentration and patience.
“We'll never, ever make any excuses, but the scheduling doesn't do us any favours when it comes to cup competitions. A lot of travel.
“They've had nine days' prep for this game, and they're a very difficult team to play against. You've got to be switched on throughout the game and not get frustrated.
“It was never going to be the prettiest of games today – I knew that. The big message to the players was stay patient, stay true to who we are, do the basics of the game really well.”
Brentford's patience eventually paid off, with Milenković turning Dango Ouattara's delivery into his own net before Thiago secured the points.
For Forest, however, the defeat leaves Glasner with decisions to make before their next fixture. His warning that changes are coming reflects growing frustration with problems that have repeatedly cost his team points.