Dortmund signed Isak from Swedish side AIK and were looking for him to take big steps in his development and break through in Germany.
He struggled to make an impact at the Ruhr side though and was even loaned out to Willem II before heading to Spain with Real Sociedad.
Newcastle snapped Isak up from the Spaniards after he got his career back on track and Kehl feels the move to Real Sociedad was one the striker needed.
“With us he wasn’t at this level yet and had this impact, but Alex always had huge potential, you could see that with us”, Kehl told German magazine Kicker.
“The timing was perhaps not the right moment to give him enough match practice to be able to develop further.
“He took a small step back and then one huge step forward.”
Dortmund will likely need to find a way to stop Isak on Wednesday evening with a visit to St James’ Park to face Newcastle United in the Champions League on the agenda for the German giants.
