Emergency crews were called to a collision in the eastbound lane of Highway 1, just east of Sydney, Man., around 4:50 p.m. Friday.
Police say the driver of a a flat deck semi-trailer had stopped on the shoulder of the eastbound lane to check his load when the truck was hit from behind by a box truck.
Investigators say it appears the box truck had partially veered onto the shoulder.
The driver of the box truck, a 34-year-old man from Portage la Prairie was rushed to hospital by STARS air ambulance with serious but non-life threatening injuries, police said Monday.
The driver of the semi-trailer, a 33-year-old man from British Columbia, was not injured.
Portage la Prairie RCMP continue to investigate.