Ritchie scored against his old club for Saturday's 2-2 draw.
Discussing the decision to sell the Scotland international, Howe told the Daily Echo: “In those situations it is very difficult for me because you have a player that is desperate to leave and join another team.
“How much you fight that and we did our best to convince Matty to stay but how much you fight that if a player's mind is made up and you keep an unhappy player and everything that goes with that. I didn't think, at the time, we had any alternative.
“We lost Matty with a very heavy heart. We knew exactly the qualities he had and how well he had done for us but it is football and players come and go and change but you never forget what they did for you. Our memories from Matty, even though Saturday happened, are still very positive."
Quizzed on whether he would handle a situation like Ritchie's departure differently, Howe said: “It depends because if a player wants to go but there is no club that wants to sign him he is not going to leave.
“If a player is desperate to go and a club are willing to pay substantial money for that player then you are put in a position where you can make the decision to keep an unhappy player but rarely, in my experience, does that work.
“Of course, it has to be right for the club and the figure the other team has to pay has to be right for the club. On this occasion it was."