West Brom dominated the Loftus Road encounter that ended in a 1-1 stalemate, and although delighted to be in the fourth round draw, Keen admits he was disappointed after Shane Long's 79th-minute strike was equalised by Kieron Dyer in the final seconds.
The two sides will now battle it out again - the fourth meeting between the two clubs this term - in 11 days' time.
Assistant coach Keen said: "We're a little bit disappointed, we felt we did well in the game, we played well, especially in the second half, created chances.
"We felt the game was going to peter out but we slipped up at the end.
"But these things happen in football and, along with QPR, we're in the draw for the next round.
"We're going to take them back to The Hawthorns and have a real go at them there. What happened in the last 30 seconds can happen in any game.
"Today it happened and an old friend of mine, Kieron Dyer, who I spent many years with at West Ham, popped up with a goal.
"We're a fraction disappointed but we've come and performed and the squad has done well considering we've picked up a couple of injuries.
"We've got a lot of positives to take from today."
Keen revealed that Zoltan Gera faces a scan after being forced off midway through the first half with a knee injury.
"Zolly had a problem with his knee," Keen added. "He'll have it scanned tomorrow and we'll go from there. We've got our fingers crossed that it's not too bad."