United announced on Monday that Greenwood will leave Old Trafford after a six-month investigation into his conduct.
Speaking at a news conference on Friday, Ten Hag was asked whether he had supported the 21-year-old's reintegration into the squad, but declined to offer his opinion.
"We are not where we want to be with our team, A lot of focus on my team so focus on the players who are available," Ten Hag said.
"I only have statements about our performance.
"I have a lot of focus on my team so I focus on the players who are available.
"We are not where we want to be with our team. I have to do everything to get the team performing."
Ten Hag was also asked whether he had spoken to Greenwood personally during the club's internal investigation, which started when criminal charges against the footballer were dropped, but the Dutchman again chose to remain tight-lipped.
"I focus on the current squad and the players who are available," he added.
The Dutchman will have to find a solution at left-back as second-choice Tyrell Malacia is also sidelined.
Summer signing Rasmus Højlund has started training with the team following a back injury but is not ready to feature against Forest.
"[Højlund] is not available for tomorrow [Saturday]," Ten Hag said.
"It is near. Today [Friday] is the first time he came into team training so next week I expect him to do a full week, all the team training. So he is very near to making the squad."