The summer signing from Reading will have surgery on his patella tendon.
Motherwell manager Stephen Robinson said: “Craig suffered an unfortunate injury through the week. It was a freak incident that happened with nobody near him as he tried a cross-field pass.
“Unfortunately for Craig, and for us, we’ll be without him for a few months. He’s been an important part of our set-up and we’ll give him all the support he needs as he works his way back.”
The 23-year-old has scored nine goals and set up another eight this season.