Standard Sport has been told by club sources that he had suffered an injury before the match and required assessment.
Alonso has now shed fresh light on his issue, saying: "At the moment has some liquid, so we have to do some tests to know how big the injury is. It's disappointing, but we need to carry on."
In his place, Geovany Quenda has been called into the Chelsea starting lineup to make his full Premier League debut.
He seems to have joined the front three alongside Morgan Rogers and Cole Palmer, with Estevao and Danny Welbeck both left on the bench.
Elsewhere, Mike Penders starts in between the sticks with Emi Martinez still recovering after fracturing his jaw away at Everton last week.
Martinez is in attendance at Stamford Bridge, and Alonso said he is progressing well in his recovery.
“It'll be a few weeks. Hopefully less than more, we have to wait but the good thing is he's feeling better and we have to listen to the opinion of the doctor and the surgeons. Considering it was a bad one, he's doing good.
“He's doing better. He had surgery and is feeling better day by day, but we still have to wait.”