Jermain Defoe: Can Sunderland afford to lose him to West Ham (or anywhere else)?

"We have never asked for a bid, we have never put a price on him. He's really important to us. He is not for sale."

And here is why the relegation-threatened Black Cats are desperate to hang on to the 34-year-old former England international...

He's scored 58% of their league goals this season

It would be unfair to describe any side as a 'one-man team', but it is clear Sunderland are heavily reliant on their top scorer - more than any other Premier League side.

Defoe has scored 11 top-flight goals this season - more than half of his team's total of 19.

No other player in the division comes close to having the same influence in front of goal.

His goals have won Sunderland an extra six points

What do Defoe's goals mean in terms of cold, hard points for his team? Are many of his strikes meaningless, consolation goals in heavy defeats?

Well, few are worthless.

Sunderland are 18th in the Premier League, inside the relegation zone but only one point behind fourth-bottom Crystal Palace.

Without Defoe's goals, Sunderland would be six points worse off - leaving them marooned at the bottom, seven points from safety.

Match-winning goals against Bournemouth and Leicester turned draws into victories, while both goals in last week's 2-2 draw against Liverpool and a late equaliser at Southampton earned potentially precious draws.

However, he is not as influential as some other Premier League strikers...

He has scored 40% of Sunderland's goals since joining

"His exceptional goal-scoring record speaks for itself."

Then Sunderland manager Gus Poyet knew exactly what the Wearside club were getting when they paid a reported £14m to sign Defoe from Major League Soccer side Toronto FC.

And so it has proved.

Defoe has netted 30 Premier League goals since the transfer in January 2015 - representing 40% of Sunderland's total tally since his arrival.

That's 23 more than left-back Patrick van Aanholt, who is the club's second-highest scorer in the same period.

Why West Ham wanted Defoe back

West Ham have struggled for consistency in front of goal this season, scoring just 23 times in their 20 Premier League matches - four of which were netted against Swansea on 26 December.

Regular injuries to Andy Carroll, Diafra Sakho and Andre Ayew have not helped matters, nor has on-loan Juventus forward Simone Zaza's inability to find his feet - or the net - in England.

So no wonder manager Slaven Bilic is keen to bring Defoe, who scored 41 goals in 105 games for the Hammers between 1999 and 2004, back to the club.

Defoe's 30 top-flight goals since joining Sunderland is only one short of the combined tally of the SEVEN forwards who have scored for West Ham in the same period.

Is a 34-year-old worth a £6m transfer fee?

Few players command a hefty transfer fee in their mid-thirties - but West Ham were clearly thinking more about their short-term strategy than long-term financial value when they offered £6m for Defoe.

For Sunderland, the maths was quite simple.

Sell the man whose goals surely represent their best chance of staying in the Premier League for £6m, then face a near £100m drop in revenue if they were to be relegated.

SAM - the Sport Analytics Machine used by University of Salford academics to predict football results - calculates keeping Defoe is worth £41.3m to the Black Cats.

It also works out selling Defoe would increase Sunderland's likelihood of relegation from 58.7% to 65%.

So is £6m worth it? The answer for Sunderland, so far at least, has been a resounding "no".