The 'Roommate' actress and the 'Avengers' actor dated briefly in 2007 but have decided to give things another go and recently rekindled their relationship.
7 September 2012 A source said: "She did a number on him back then. But she reached out to Chris this time."
However, Chris wants to take things slowly as he doesn't want to get hurt again.
The source added to Us Weekly magazine: "He's still a little insecure, so he's being cautious."
Chris - who has previously dated Jessica Biel - recently said he wants a girlfriend who isn't afraid to argue with him and is not bothered by his celebrity status.
The 31-year-old hunk said: "I like ... girls who give me a hard time [and] bust my chops a little. I like girls who aren't so la-di-da. L.A. is so la-di-da. I like wet hair and sweatpants. I like sneakers and ponytails."
Mika, 32, was recently liked to actor Jake Gyllenhaal after the pair went on a string of dates together and was previously in a long-term relationship with Derek Jeter.
