The 31-year-old suffered the injury in last Tuesday's first-leg victory in Lisbon.
He said after the 2-1 win at Stamford Bridge: "I couldn't breathe and my chest was closing up.
"I was having a difficult time out there, especially in the first half. Hopefully, I'll be okay. I don't know how long I'll be out. I will have a scan in the morning and we will see where we go from there. It's just horrible.
"When you get one in the ribs, you can't do anything about it, you can't treat it, you just have let it heal, but I can get through games definitely.
"I thought it was okay but, later in the second half, it just seemed to be getting worse and I was struggling to breathe to be honest. It was difficult. I have never felt anything like that before."