Simmons played well for the most part, so it's hard to question the decision to start him instead of Pocic. His run blocking looked pretty good, and he was anchoring well in pass pro. He did allow a costly sack on 3rd down in the red zone. But considering the strength of the opposition and the fact that it was his first NFL start, I thought Simmons held up well.
Pocic was also playing well before his injury; he had the highest run blocking grade on the team during that span. Starting Pocic probably would have meant moving Sweezy back to RG, so Pocic could play LG again. That's a change in continuity that would not have been desirable. And then Hunt would have been needed as the backup center, which would have made them vulnerable to injury at the guard position (where Hunt has struggled as a backup). Pocic can play all five positions, which gives him huge value as a backup. It's also likely that he is still hindered somewhat by his injury.
If they needed Pocic to play mid-game due to an injury, I feel pretty confident that he would play well.