I ask because on the one hand he and some of Pharoh's other servants are described in the Hebrew as סריס, but the translations I've seen are usually in the vein of Lord Servant. For instance, my IDF Tanach lists in it's footnotes סריס as just a synonym for שר. Hirsh does mention in his commentary that this is one meaning of the word, but doesn't use this meaning in his translation.
Translating סריס as Eunuch would seem to explain allot of things. From what I've read it was common in many societies for royal Eunuchs to marry and adopt children. This would maybe explain why Potiphor's wife was so after Joseph. Also, I've heard that Osnat was Jewish. If Potiphor was a Eunuch and she was adopted this would make sense.
Anyways, why doesn't this translation seem to be used, especially as this is the word's literal meaning and it makes good sense in the context of the story.
Edit
I wanted to add one more thing which I think pushes in the direction of Potiphor being a eunuch. As far as I've noticed, all the servants of Pharoh mentioned in the parasha who would have served Pharoh in person such that they might have regular personal contact with him such as Potiphor, the cupbearer and the baker, are all described as both שר and סריס. However the lord of the jail, who would not have direct contact with Pharoh on a regular basis is described only as שר. This would seem to be in line with what we know about ancient practices.