"Our thoughts are with him because he has worked so bloody hard to get back and now we just hope it is only a little tweak and that he is only out for a few weeks," Crusaders coach Scott Robertson said.
Dagg, who has played 66 tests, hasn't played for the All Blacks since suffering his knee injury while playing against Argentina in New Plymouth last September and desperately needed a run of games to be in contention for the three tests against France in June.
With Nehe Milner-Skudder making a comeback and joining Waisake Naholo, Jordie Barrett, Ben Smith and Rieko Ioane as an outside back option, Steve Hansen has no shortage of talent and Dagg would have been an unlikely inclusion anyway, but this latest problem sets him back further.
A dislocated shoulder in 2015 put paid to Dagg's World Cup dreams after he starred for the All Blacks at the tournament four years earlier and the Hawkes Bay player has had a history of calf problems. In early 2011 he tore his right quadriceps off the bone in a freak injury while clearing for touch in a match against the Stormers in Cape Town.
His desperation to get back on the field this year can be seen in his trips to Queenstown where he got stem cells injected into his right knee.
"Hopefully that works, and apparently it regenerates cartilage and that is what I need," he said after his comeback game for the Crusaders against the Sunwolves last week. "I need cartilage in between my bones, because pretty much it is just bone on bone.
"I think it is helping, it is doing its thing. At the end of the day I just have to get my body strong and the thing with this injury is that the muscles can switch off. So I have to really fire my quads up, get them strong. Without them, then it is pretty much my knee doing all the work."
Warming up for training or a game takes 45 minutes. He has played at the highest level in the biggest stadiums around the world but was so nervous before his first comeback game for his University club he couldn't eat.
He said last week he wasn't the player he used to be, but he just wants to play, and the vast majority of rugby supporters, no matter their allegiance, can probably sympathise with that.
To get the day's top sports stories in your inbox, sign up to our newsletter here