According to HTC-Highroad directeur sportif Valerio Piva, Lewis has been diagnosed with a fractured femur, while Pinotti injured his knee and has a suspected fractured collarbone. Both riders were taken to the nearby hospital in Domodossola, not far from the stage finish in Macugnaga.
About an hour after the stage finishes Pinotti sent a reassuring Twitter message: "At the hospital in Domodossola. Just spoken with @craiglewis85. Just want to say he is conscious and quite fine. We will be rooming together."
The two riders hit the traffic sign while the peloton was racing at speed and in pouring rain some 45km from the finish. Italian television showed Lewis lying on the road after hitting the straw bail that protected the sign. The sign divided the road into two but it seems there was no marshal warning the riders of the danger.
Lewis was treated by the race medical staff, led by Dr Giovanni Tredici.
"We found three riders on the ground when we stopped. Two were from HTC-Highroad and one was from Androni Giocattoli," he told Italian television.
"The Androni rider was okay and quickly got back up but the other two were taken to hospital because the seemed to have important injuries."
Both Lewis and Pinotti were hoping to finish the Giro in Milan on Sunday. Pinotti finished second on the stage to San Pellegrino Terme on Thursday and was targeting the final time trial to Milan after being the first pink jersey wearer of this year’s race.
Lewis played a key role in HTC-Highroad's success in the opening team time trial and worked hard to help set up Mark Cavendish for his two sprint victories.
Britain's Russell Downing (Team Sky) also crashed during the stage. He was helped back up to the road after tumbling deep into the trees on the descent of the Mottarone. He got back on his bike, crashed again, suffered a puncture but still finished the stage.