The 22-year-old centre-back joined Celtic from IK Start in 2016 and has emerged as one of the top players at Parkhead over the last few seasons.
He has been on the radar of a number of clubs in recent transfer windows, but Celtic have kept hold of him and it remains to be seen when he will depart Glasgow.
Solbakken stressed though that it is time Ajer finds a way out of Celtic as he is not facing enough challenges as a defender on a game-by-game basis in Scotland.
He insisted that some of his shortcomings are only being highlighted when he is playing for Norway or in Europe with Celtic against better teams.
Solbakken told Norwegian daily VG: “He has to change the league now, he must.
“He does not have enough challenges in everyday life.
“Therefore, I think he has got some habits, which you do not notice until he meets better opposition at international level or Europe.
“So, I think it should be a smart move for him.”
Solbakken insisted that Ajer is good enough to play for one of the clubs in the top five leagues in Europe.
“He should go to one of the five major leagues. He will do that.
“He has the qualities to play in those leagues and I am calm about that.”
Ajer has won three Scottish league titles during his time at Celtic.
