BOSTON — The New England Patriots placed the franchise tag on Matt Cassel Thursday, making it tougher for other teams to sign the free agent quarterback.

The Patriots might keep him if they have doubts that Tom Brady will be ready for the start of next season after sustaining a serious knee injury in the 2008 opener. If they're confident Brady will be ready, they could trade Cassel rather than pay him the $14.65 million offer for 2009 in the franchise designation.

For now, he remains a free agent, but the Patriots can match any offer made by another team or allow him to sign with that team in exchange for two first-round draft picks. The franchise tag value is the average of the top five paid NFL players at a particular position.