The 25-year-old left the field on a stretcher following the sickening clash of heads in the first half of Sunday's game at Stamford Bridge.
Mason got clattered going up for a header with the Chelsea defender, who also required treatment.
Cahill was able to carry on, but the Tiger has now been taken to St Mary's Hospital for closer inspection, accompanied by the club doctor.
Manager Marco Silva said: "I imagine in this moment that he will stay in hospital. I hope nothing special, nothing serious, but we'll see.
"I don't know (if he is awake).
The doctor told me he was in the hospital and we await news.”
The injury led to nine minutes added time at the end of the first half, with Diego Costa giving Chelsea the lead with 90 seconds remaining before Cahill wrapped up victory after the break.
Blues midfielder Cesc Fabregas said: "First, I and all the Chelsea players want to send our best wishes to Ryan Mason."