Howe suspects ankle ligament damage for Defoe
The England striker attempted to play on following a heavy early challenge that earned Ethan Ampadu a yellow card, but was ultimately forced off in the 2-1 defeat at Stamford Bridge.
Defoe will almost certainly miss Saturday's trip to Premier League leaders Manchester City and will be joined on the sidelines by Harry Arter, who picked up a calf complaint.
Speaking at his post-match news conference, Cherries boss Eddie Howe revealed his concern over the extent of Defoe's injury.
"Potentially [he's done] some damage to his [Defoe's] ankle ligaments," Howe said.
"Arter hurt his calf in the second half, so we are really stretched. He will be out this weekend.
"Personally, I did not see the challenge on Defoe as a red card. My gut feeling was a yellow, but obviously we lost Jermain."
Bournemouth appeared to have pushed the tie into extra-time when Dan Gosling curled home in the 90th minute to cancel out Willian's first half opener.
But Alvaro Morata broke through the heart of the visitors' defence from the resultant kick-off to restore Chelsea's lead at the death.
75 - There were just 75 seconds between Bournemouth's equaliser, and Morata restoring Chelsea's lead, with the only two shots on target in the second half of #CHEBOU. Rapid.
— OptaJoe (@OptaJoe) December 20, 2017
"It is heart-breaking. We penned them in with waves of attacks," Howe said.
"It has been a bizarre week for us. We went to Manchester United and were really proactive, [the 4-0 defeat to] Liverpool was poor, then we played so well here.
"We leave positive, but we have Manchester City next."