Fulham director of football Tony Khan has promised to sign defenders following Tuesday's 3-0 loss to Aston Villa.
The Cottagers back-line looked desperate in the home defeat to Villa.
Sassuolo stopper Marlon Santos failed a medical with Fulham this week, but Khan says the club will not stop looking for more defenders.
"I apologise to Fulham's supporters for our performance tonight," Khan said.
"We've looked to add centre-backs since Wembley, I'm sorry we haven't yet as two got COVID and we lost a Free we thought was close and had another issue with a fourth centre-back.
"I promise players in and better efforts from this squad.
"I should and will apologise repeatedly for that performance. I'm sorry everybody. We all need to do a better job.
"Everyone at the club for the past year worked really hard to get the team up, & now we need to work significantly harder to stay up. I promise better efforts than today."