Bale returned to Spurs on a season-long loan deal in September amid huge fanfare, but he has failed to live up to the hype thus far.

The 31-year-old has only made one Premier League start and is yet to play a full 90 minutes in any competition amid issues with injuries, fitness and form.

After reports suggested Spurs had already decided not to extend his loan for a second season, manager Jose Mourinho insisted last week there has not been "one second of discussion" over the issue.

But former Spurs star Berbatov believes the outcome is clear unless the Wales international embarks on a rapid turnaround in the second half of the season. 

Berbatov said: "I was expecting a lot of him coming to Spurs, to shake things up and for him, Son and Kane to make it a trio up front that can rival the Liverpool attacking trio, for example.

"For the moment it is 100 per cent not working.

"There are still games until the end of the season, enough for him to show what he can do, in the [EFL Cup] final they play as well.

"So hopefully he can get back to 100 per cent and start playing. If not, in the end it is an easy decision – he can go back to Madrid."

Bale was an unused substitute in the 3-1 win at Sheffield United on Monday (AEDT).

Berbatov said: "In a way, it is disappointing that he is not being given more time to play. 

"But if you are not given more time to play, there are two reasons for that. Either are you injured and not in good shape to play or you are not performing well in training.

"Whatever the reason [for his lack of games] Gareth Bale is injury free now is because he's on the bench for the team.

"But he's not playing from the from the first minute so it means that obviously he's not producing the right level of football Jose Mourinho wants from him, which is a shame because I like him a lot."

With the EFL Cup final against Manchester City coming up in April, Berbatov feels Bale – along with Mourinho and captain Hugo Lloris – can play a huge role. 

Spurs were unable to win a trophy despite five and a half otherwise productive years under Mauricio Pochettino.

Berbatov – who won the League Cup with Spurs in 2008 – underlined the importance of lifting silverware.

"Jose Mourinho has led them to the final but let's not celebrate now because you need to make one more step," said Berbatov.

"I remember the game before we qualified for the final. We played against Arsenal and we beat them 5-1, everybody was celebrating like we won the final. And I'm like, 'People we are not there yet'. 

"We need to wait, we have one more step to make and then we can celebrate."

Berbatov continued: "The person who brought them there is Jose Mourinho. I think this is the reason they brought him into the club – to win trophies.

"It doesn't matter how [they get there] – the Premier League title, the cup, something – they need one so they can get that winning mentality going. 

"Although they have a world champion in their team, the goalkeeper, he should know how things are won – especially [having lifted] the World Cup. 

"So him, Jose Mourinho, Gareth Bale – who is coming from winning mentality – will run with it. Those three are very important, they can teach them in a way how to win things and then get that winning feeling.

"And after get that feeling, trust me, you don't want to let it go."