The 30-year-old midfielder, who had been without a club since leaving QPR in the summer, made his full Pompey debut in Tuesday night's disappointing 3-1 Johnstone's Paint Trophy defeat by Wycombe.
He said: "I'm not there yet, but the game was important for me. I don't expect to be hitting top form after one-and-a-bit games.
"I will get better and I will get sharper - I'm confident that I will show the Pompey fans that I'm good enough. I want to move forward with this club.
"We played some good football against Wycombe, but made some big errors. If you make mistakes at any level, you're going to concede goals."