The Cruel Intentions star dated Knapp for several months last year (10) before splitting in September (10).
Knapp recently stepped out in Hollywood with a distinct baby bump and, although she has yet to officially comment about her pregnancy, sources tell UsMagazine.com she's convinced Phillippe is the dad.
The insider says, "She's being telling people he is the father. Ryan doesn't know if he is the dad. If he is, he will take full responsibility of the child."
The actor already has two children, Ava, 11, and Deacon, seven, with his ex-wife Reese Witherspoon. The couple divorced in 2007 and Phillippe has most recently been linked to Mamma Mia! star Amanda Seyfried.
Representatives for Phillippe and Knapp had yet to respond to requests for a comment as WENN went to press.