K en Morrow thought about a lot of things last Wednesday night. He thought about his daughter, Krysten, who will soon spend her first full summer in the world. He thought about the things his wife and daughter will do together. Morrow remembered the golf game that he will soon have the opportunity to improve. He imagined the possibility of a fourth Stanley Cup celebration in his fourth season with the Islanders. He looked forward to his job as an assistant coach during the tryouts for the United States Olympic hockey team at the end of June.

He was thinking of all these things in an effort not to think about what was happening at the moment. ''But it's hard to be thinking about other stuff,'' he said, ''when they're doing something to your knee.''

While Morrow's teammates were in Boston late Wednesday, he was lying on a table at Lenox Hill Hospital in Manhattan, as another piece of cartilage was removed from his right knee. ''I was staring at the clock the whole time,'' he said. Less than 70 hours later, Morrow was on the ice at Nassau Coliseum, his presence helping the injured Islander defense in the sixth-game victory that ended the semifinal playoff series against the Bruins.

Morrow, the beneficiary of modern medicine, was on the ice a second more than 25 minutes Saturday night. He is fortunate to be a National Hockey League player of the 1980's, playing when his two most serious injuries - which not long ago would have ended his season - have been treated and rehabilitated in the same period of time that it takes to treat a case of the flu. He has had two arthrocopic procedures on his right knee -the one last week and the first in 1980, his rookie season.