United sealed the third place, finishing the season with 75 points, after they came from behind to defeat Fulham 2-1 in the last game of the league season on Sunday.
The Old Trafford club, which won the League Cup in February, has the chance to add to their silverware when they face Manchester City in the FA Cup final on Saturday.
"We are in the right direction, but we are not there where we have to be, there's still a long way to go, there's potential in this team and individual players," Ten Hag told reporters on Sunday.
"We showed during the season we made progress, that's a compliment to the players and the coaches, we worked really hard, but we have to make an investment.
"The club knows if you want to play top four, compete for trophies in this tough league then you have to invest otherwise you don't have a chance because other clubs will do."
United last won the league title in 2012-13 under Alex Ferguson.