教授这是什么意思呢？

我问：A few schools I'm applying to require only 1 recommendation. I'm inclining to list you as the sole reference for these programs. What do you think?
Prof. 说：better to get a letter from D （我的博士导师，关系不大好，而且改行让博导知道，博导怎么想啊）. I can write a rec that she can quote in her letter. What do you think?

这教授是啥意思呢？觉得自己写的不会太strong？不想被只有1封推荐信申请失败的情况下暴露了自己推荐信写的不strong？还是啥呢？