Mainz chief Christian Heidel says their league position has influenced the decision to keep hold of Shinji Okazaki.
The Japan international has been the subject of two concrete offers from Leicester City this month.
But Heidel says they cannot afford to lose the forward midseason.
"People always say: Sell because then you'll get someone worth twelve, 13 million. But I cannot see getting a striker who can guarantee me that he scores like Shinji. This risk was too great for us.
"This shows that we do not always have to sell. If we had been on 25, 26 points, the decision might have been different."