I have had both of my Achilles tendons re attached and and had issues with blood clots so can talk from experience. When you have leg surgery and are immobile for a few weeks/months you may get clots. Generally you need to have blood thinning injections or medication to try and prevent clots while recovering. If you had your leg in plaster for a period of time , when they take it off they will send you for an ultrasound to check for any clots in calves, If there is it can be serious if they travel past the knee. They can end up in your lungs, brain or heart . Generally if you have clots you will need to be on blood thinners and checked regularly to see if they are reducing, if not then they may need to open the veins to remove the clots.