Oakland Raiders head coach Jon Gruden said Tuesday he's surprised quarterback Colin Kaepernick remains unsigned for a second straight season, but added he believes it has to do with his on-field play.

Kaepernick spent all of last season as a free agent, with many pointing to his national anthem protests in 2016 as the reason he remained unsigned.

"I think there's a lot of intrigue there," Gruden told ESPN. "His performance on the field wasn't very good, on tape. I think, Robert Griffin, a rookie of the year, [I'm] surprised he's out there. Tim Tebow takes a team to the playoffs, there's some surprise that he never came back. You know, Johnny Manziel, he's out there.

"Back to Kaepernick, he got beat out by [Blaine] Gabbert to start the [2016] season. I think that says something. [But] I am surprised he's not in camp with somebody. He probably will be soon."

The 30-year-old spent parts of five seasons starting for the San Francisco 49ers and led the team to the Super Bowl in 2012. He lost a quarterback competetion to Gabbert to begin the 2016 season, but went on to start 11 games. Despite a 1-10 record, Kaepernick threw 16 touchdowns to four interceptions in 12 games posted his highest passer rating since the 2013 season. 

Gabbert signed with the Titans on Tuesday.

Kaepernick has 12,271 passing yards, 72 touchdowns, and 30 interceptions in six NFL seasons.