Dean Smith confirmed Brentford opted to cash in on David Button, rather than let the goalkeeper run down the remaining year on his contract and leave for nothing.
The 27-year-old agreed a deal with Fulham, who scraped to 20th position in the Championship last season, with the Bees, who finished ninth, benefiting from a seven figure fee for the stopper's services.
It is the second time this summer the club have sold a player to a local rival with captain Jake Bidwell joining QPR.
And Smith, while understanding the rivalry between Brentford and Fulham, urged fans to think of the economic reasons behind the decision.
He said: “You have to look at the economics of football. Do you let them run down their contracts and let them go for nothing or let them go and invest in the squad.
“I've got the utmost respect for both players but they chose not to sign contracts.
“It's one of those things that happens in football. We wanted David to sign a contract and he's not committed his future to us.
“We had to make a decision and David had to make a decision. We didn't want him to run down his contract and go for nothing.
“We know how valuable David is but he wanted to see what was out there as well.”
Brentford already have a replacement for the goalkeeper in Daniel Bentley an Smith has also been impressed with Jack Bonham's displays in pre-season.
He said: “I think Daniel Bentley has the number one shirt in terms of squad numbers but I want Jack to push him all the way.
“He knows he's got a clean slate. People have spoken to me about games he has played in before but all I can do is judge him on what I've seen.
“Against Boreham Wood and Wycombe, his distribution was very good. He knew he couldn't play out at times and he had to drop it into the frontman.
“His distribution was very good. He made one outstanding save when the lad was offside.”
