My first question was the same as your first question.
What is a Soleus?
Lets go to Wikipedia.
In humans and some other mammals, the soleus is a powerful muscle in the back part of the lower leg (the calf). It runs from just below the knee to the heel, and is involved in standing and walking. It is closely connected to the gastrocnemius muscle and some anatomists consider them to be a single muscle, the triceps surae. Its name is derived from the Latin word, “solea,” meaning “sandal.”
So in essence it is a strained calf, it would have been much cooler if it was something of more comedic value, but what can you do.
He should be good to go when the playoffs start hopefully for the Spurs.