The 25-year-old, who had been the team's third-string quarterback at the start of the season, will start this weekend's game against the Kansas City Chiefs.
Keenum told reporters: "It was something that I've always wanted. It's something that I've worked for for a very long time and to be in the same city that I've been in for the last six or seven years, it makes it even more special.
"I have to pinch myself every now and then. It's exciting. It's what I wanted, what I've wanted for a long time, what I've worked extremely hard for. What a lot of people don't know that I've done, there are a lot of stuff out there, there's been a lot of time and a lot of effort and sweat coming back from injuries.
"It's a small, little celebration I guess you could say, but it's on now. It's time to go work. It's time to go help this team. It's a big one this Sunday and I'm excited to go out there and do that."
Starter Matt Schaub will miss the game against the Chiefs on Sunday with a leg and ankle injury, while backup TJ Yates threw two interceptions, was sacked twice and fumbled during last week's loss to the St Louis Rams.