Watson entered the league after scoring a 48 on his Wonderlic Test. That is tied for third highest score ever in the NFL. He was drafted by the New England Patriots 32nd overall in the 2004 NFL draft. He was on the injured list for almost his entire rookie season for the Patriots, playing in only one game. Watson received a Super Bowl ring following the Patriots' win over the Eagles in Super Bowl XXXIX.
He continued to battle injuries throughout his stint in New England, starting just 47 games in six seasons with the team. He nabbed a career-best six touchdown passes in the Patriots' undefeated 2007 campaign, third best on the team.
On March 12, 2010, Watson signed a three-year deal with the Cleveland Browns, reuniting him with former Patriots assistant coach, Eric Mangini. In the 2010 season, Watson led the Browns in receptions, receiving yards, and receiving touchdowns (tied with Brian Robiskie).