The goalkeeper was left dizzy and with a bleeding ear following a clash with Alexis Sanchez during his side's 2-0 win over Arsenal, but was initially cleared to play on, before being substituted 13 minutes later.
The 22-year-old told Sky Sports News: "The doctor of Chelsea did well, I think. She asked me the questions she needed to ask and I could repeat immediately so I was feeling well and didn't have to come off.
"I didn't feel I was knocked out but I was hit. I didn't have a headache or anything, but after a few minutes I felt worse and worse. Then I started to feel a little dizzy and I couldn't see the ball anymore, and there was a little blood coming from my ear. Because I saw trouble, I went off.
"I know you have to be careful, but I was not really knocked out. I had some neck pains, dizziness, and headaches for a few days afterwards, so that's why I stayed in England. I had a little confusion I think, and a little blood. I had neurological tests to see if my brain was responding correctly and, after it was okay, I travelled to Belgium."
Courtois was able to play all 90 minutes in Belgium's 6-0 win over Andorra in their Euro 2016 qualifier on Friday.