Real Sociedad is one of the big teams in La Liga. They often finish the season in the top 6 of La Liga standings. I agree if we can't underestimate Real Sociedad because they have the capability to make a surprise. We know if the teams from La Liga isn't an easy team to be defeated. They often win international competitions like UCL and UEL. So, PSG must be careful to play against Real Sociedad in the upcoming match. Moreover, if Real Sociedad can play with full team. They have the potential to defeat PSG and qualify for the next round. However, I still trust PSG to win the match because they have more quality players.
Indeed Real Sociedad is not favorite teams in Champion League but success finish on 1st standings and made Inter Milan as top teams losses their first standings from Sociedad as mediocre team. Facing PSG on 16 round is not easy for Real Sociedad but their achievement finish on first standings group stage will be motivate for defeating PSG exactly have poor performance in group phase. PSG performance not quit well in group phase they have losses 2 of 6 matches than Real Sociedad with unbeaten from 6 matches, I think could be motivate for Real Sociedad with other teams such as Newcastle United and AC Milan success defeated PSG on group phase round.
I don't optimistic with PSG chance for defeating Real Sociedad in 16 round, have excited match and not easy give first loss for Sociedad in Champion League match.