Could implies that you had the freedom to be able to choose or do something.

Example: I could pick up the phone.

In this sentence it doesn't specify, did you pick up the phone or not?

however if you change it to able to

Example: I was able to pick up the phone.

 This actually specifies that you did in fact pick it up.

To better explain this in case I was not in depth enough I will quote an answer by Arun in the website english.stackexchange.com in which he uses the book <em>A Practical English Grammar</em> by A. J. Thomson.

link:http://english.stackexchange.com/questions/122310/was-able-to-vs-could

"1)According to 'A Practical English Grammar' by AJ Thomson et al, the one difference between could and be able to in the past affirmative is that could just implies that you had the general freedom or permission to do something, but may not have actually done or achieved it, where as be able to implies that you actually went ahead and did it.

2) Also, ** could ** does not have to be necessarily about a past event, it can also be used as a supposition of ability in the present. Was able to clarifies that it is indeed in the past.
Eg: He could save his parents from the sinking boat.
It is not clear whether this is past or present. If it is present not clear whether he actually did save them. He was able to save his parents
This emphasizes that he actually made an effort and succeeded in saving them.
So, I think in your example if the idea is to say that there was an expectation or permission that he could return to work, then it is OK. But the intended meaning is that he was well enough to do that, then we should use was able to."

Thank you so much for helping me with my English question.