United are looking to bring in Burnley's Netherlands striker Weghorst on a deal to the end of the season, but first had to secure his exit from a loan at Besiktas.
Erik ten Hag on Friday said a deal is "close" for a player who has nine goals in 18 appearances in Turkey, but the manager confirmed United had not got it over the line before the 12pm registration deadline ahead of Saturday's match.
That could leave Ten Hag with a problem as Martial is struggling with a leg injury which kept him out of training during the week, although he was able to take part on Friday morning.
"It is a small thing in his leg, so it will be better tomorrow or he will be available for Wednesday," Ten Hag said of the Frenchman.
If Martial is injured, Ten Hag could be forced to shift the in-form Marcus Rashford - who has scored seven goals in his last six games - in from his preferred position on the left, although Ten Hag played down the potential impact.
"We have won a lot of games without Anthony Martial," he said.
Diogo Dalot is out of Saturday's match with a hamstring injury, while Ten Hag revealed on Friday that Donny van de Beek's season is over following the knee injury he suffered in a collision with Bournemouth's Marcos Senesi on January 3.
"On the long term it is looking really good, he will be back for the start of pre-season," Ten Hag said of the £35million midfielder. "The rest of the season he is out.
"I think everyone's thoughts are with him. It is terrible when you are in a season and you get a bad tackle, a bad injury. It is always really disappointing and thoughts, feelings are with the player, you express that, our group did that really well and we are now happy his knee in the long term is good.
"In the same moments, mixed feelings. Disappointment he is out of this season. The players are feeling we are building something good and we want to be part of it and everyone wants to be part of it."
United were beaten 6-3 when they last faced City in October. But, since then, Ten Hag's side have lost only once in 18 games, winning their last eight to stay alive on four fronts and climb into the Premier League's top four.
Asked how far United have come since they were humbled at the Etihad, Ten Hag said: "We have to prove that on the pitch. There is only one truth and it is always on the pitch.
"It is not coming overnight, there has been a process in the last months where we develop, progress. We have to be brave, act with belief and play our game…
"We took the lesson (from the last meeting) and from that point on we make huge progress. We have to continue that process."
Key to the turnaround in United's fortunes has been the arrival of Brazil midfielder Casemiro. The 30-year-old was a second-half substitute in the last derby as he was still settling in after his move from Real Madrid, but has since established himself as a key cog in United's engine room.
"He's so important," Ten Hag said. "He's the cement between the stones. His defending, his organisation, he knows his position, anticipating and pointing players into the right position, winning balls in midfield.
"He has that competitiveness to give to teams at the top level. There are not many players of this quality in the world. He is one of them and we are really happy to have him on board."