Goffin is a very dangerous player, but his stats are grossly inferior to Thiem in 2017.
Thiem is up for the year through today to 42.1% return points won up from 40.0% in 2016. On serve Thiem is up to 66.0% from 65.4% last year. Total points won is 54.1% up from 52.7% for 2016. To put 54.1% in perspective, Roger Federer is 53.6% points won on clay for his career and Novak Djokovic is 54.0%. They are 2nd and 3rd all time behind Nadal at 56.0%.
This kind of level is easily good enough to win slams, but with prime Nadal back on the loose in 2017 Thiem likely would need help from the field in disposing of Nadal.
Thiem is very, very dangerous even to Nadal because of his first serve points won. On average Thiem is 73.0% first serve points won. Federer is the king of this stat among successful clay court players with 74.5%. Kuerten was 73.8%. What makes Thiem extra dangerous is that rather than be at 60% first serves in or 55% first serves for Kuerten, Thiem averages more first serves in play at 63% so far in 2017. Thiem just had matches with 73% and
a whopping 84% first serve rate. If he does that near the end of a tournament Thiem would be able to challenge prime Nadal.
Serves win slams because a strong serve allows a player to get efficiently through the draw. So far in 2017, Thiem has been straight setting most of his opponents which will allow him to be his very best deep into tournaments.