Can an offer letter create liability?

Q. Our company recently terminated a manager who had been with us for less than three months. He just seemed not to be the right fit. Now the former employee is threatening to sue, saying he left a good opportunity to take a job with us, based on our offer and what was said in the hiring process. We did use an offer letter, which stated that employment would be at-will and that the offer letter did not constitute a contract of employment. Do we have cause for concern?