Bale was forced off in the second half at Craven Cottage after suffering a hamstring injury as he stretched while shooting at goal.
Tottenham also lost central defender Michael Dawson with a similar problem after only 15 minutes.
Bale is expected to miss Tottenham's next two games, but Dawson could be back for next weekend's Premier League clash at Everton.
Striker Defoe, who scored twice against Fulham, said: "Obviously losing Gaz is a massive blow for the team.
"Because of the way he plays you can't really replace a player like that, but at the same time I think someone else can come in.
"We're lucky because we've got some great players that can come in, play out of position, adapt and do a job for the team.
"When Gaz went off the pitch Clint Dempsey went over to the left-hand side and did well. I think we will be okay."
Defoe has now scored 12 goals this term to put him on course to beat his record of 24 goals in a campaign.
"When you're scoring goals, you're buzzing. You just can't wait for the next game because you've got that belief," he said.
"You always feel like you're going to score every game you go in to, because you're scoring goals.
"You know you're going to play, you know you're going to stay on the pitch, so I just stayed focused and took the chances when they came."