United are sixth in the Premier League standings with 54 points, 13 fewer than fourth-placed Aston Villa and six below Tottenham Hotspur in fifth place. The Old Trafford side have four matches left in the league season.
Ten Hag flagged that United had suffered an alarming number of injuries, but they would still need new recruits after falling short of expectations.
"(There's) always a lot of work, it's normal when you go into a transfer window, but the injuries had a big impact," Ten Hag told reporters ahead of Monday's Premier League clash at 14th-placed Crystal Palace.
"We know also we are still below the levels from the expectations Man United has so we have to catch up and I'm now going into my fifth window - two windows quite okay, two we missed - so we have to make new improvements to the squad.
"But here is a base of this squad that is very good."
Midfielder Bruno Fernandes is a doubt for the trip to Selhurst Park, while forward Marcus Rashford's availability is still unclear.
Rashford picked up an injury in the FA Cup semi-final against Championship side Coventry City, which United won on penalties after surrendering a 3-0 lead at Wembley. The forward, 26, has since missed the games against Sheffield United and Burnley.
"Scott (McTominay) didn't train so far this week but I expect him back in training on Saturday and we have to see if he's 100% available or for a part of the game," the Dutchman said.
"After the game (Burnley), we had an issue with Bruno. He is a doubt for Monday. We all know Bruno, never rule him out for any game. He will do his best to be available.
"Jonny (Evans) returned today for team training. Tomorrow we are off and then two days until the game. (Anthony) Martial was partly in team training.
"I can't tell (about Rashford). His issue hasn't healed so far."
Ten Hag said he was happy to see Jadon Sancho, who is on loan from United at Borussia Dortmund, thrive. The 24-year-old winger produced a fine performance in Dortmund's 1-0 win in the first leg of their Champions League semi-final against Paris St Germain on Wednesday.
The England international joined United from Dortmund for 85 million euros in August 2021, signing a five-year deal, but he failed to hold down a regular spot and returned to Germany after becoming embroiled in a row with Ten Hag.
"So let's say this: yesterday he played very good and he's a very good player. Yesterday he showed why Man United bought him and he showed he represents a high value for Man United, which is good," Ten Hag said.
"I'm happy for Jadon, for the performance yesterday, and we'll see what is going to happen in the future."
