almost certainly “几乎可以肯定”，是很正确、很地道的说法。比“可以肯定” 的程度是低了一些，为说话人下结论留有余地。有例句为证：

When the final numbers are tallied, sales will almost certainly have fallen.

等到最终的数字统计出来后，几乎可以肯定销售额下降了。

《柯林斯高阶英汉双解学习词典》

The bombs are almost certainly part of a much bigger conspiracy

几乎可以肯定，这些炸弹是一个更大的阴谋的一部分。

《柯林斯高阶英汉双解学习词典》

It would almost certainly mean the end of NATO 

这几乎肯定会导致北大西洋公约组织的终结。

《柯林斯高阶英汉双解学习词典》

The arrested man will almost certainly be kept at this police station

几乎可以肯定，被逮捕的男子将被扣押在该警察局。

《柯林斯高阶英汉双解学习词典》

This was almost certainly the high-water mark of her career 

这几乎肯定是她事业的巅峰。

《柯林斯高阶英汉双解学习词典》