During the early stages of their UEFA Nations League showdown at Den Dreef, Greenwood was involved in a worrying head collision with Belgium striker Jassina Blom, seeing both players immediately drop to the turf.
Play was halted for over 10 minutes as medics attended to the stricken pair, although Blom eventually came around and was able to walk off the field after being patched up.
However, Greenwood remained on the floor for several moments before being loaded onto a stretcher, and she was also given oxygen as she was carried off the field to applause from the Leuven crowd.
Jess Carter came on for the 30-year-old, who was soon confirmed to have been conscious and talking to staff, while Blom was controversially allowed to continue after undergoing assessment off the pitch.
Without Greenwood on the pitch, England conceded three goals under Wiegman for the very first time, as a frenetic opening period saw Laura De Neve, Lucy Bronze, Fran Kirby and Tessa Wullaert make the net bulge.
The Lionesses failed to take their chances in the second period, and they were made to pay in the 85th minute, as Wullaert sent a powerful penalty past the outstretched arm of Mary Earps following a Georgia Stanway handball.
When asked about Greenwood's condition after the game, Wiegman revealed that the defender had sustained a concussion, but she was able to walk around, saying: "She's fine," as quoted by The Guardian.
"I haven't spoken to her yet. I also haven't spoken to the doctor about it yet, but I heard she's walking around. Obviously she has a concussion, but it's good news that she's walking around and doing fine."
England's defeat in Leuven - which came four days after they had overcome Belgium 1-0 at the King Power Stadium - leaves the Euro 2022 champions' hopes of qualification for the Paris 2024 Olympics hanging by a thread as they sit third in League A Group 1 of the Nations League.
The Lionesses are one point adrift of Belgium and three worse off than the Netherlands with two games remaining, and as things stand, England will be condemned to a relegation playoff as the Dutch advance to the Nations League finals.
Alongside hosts France, the two teams who reach the final of the Nations League will earn a ticket to Paris 2024, but Wiegman has insisted that she is not pressing the panic button just yet.
"We're not panicking or anything. You could see that we were trying hard, we were trying to find the space and we were trying to have different crosses and play a possession game into the final third, but we just missed in the final perfect touch... although we did create chances to really score goals," the 54-year-old added.
"There were a couple of things we wanted to do better today, but in the end we were too sloppy. They had more counterattacks than they did on Friday, so they did that well and I think we could have done better."
England face a crunch match against the Netherlands at Wembley on December 1 before rounding off their group-stage campaign against Scotland at Hampden Park four days later.