The Denmark international spent a year in the Italian capital on loan from Wolfsburg, but started only 20 Serie A matches as Roma struggled under Luis Enrique, eventually finishing seventh in the table.
The defender concedes that it was a troublesome period for both him and the club, but that his season at the Stadio Olimpico helped him grow as a player.
"The year I went to the capital was a particularly difficult time for the club," he told EuropaCalcio.
"There was a change at the top and a lot of the staff changed as well.
"I think I played about 20 times for Roma. Summing it up, it could have definitely gone better.
"But sporting director Walter Sabatini definitely furthered my career and helped me grow as a professional, so I thank him for that."
Lille and Roma are both vying for UEFA Champions League qualification, with the French side third in Ligue 1, while Rudi Garcia's men are second in the Italian top flight.
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