The £30.75million forward has found first-team places difficult to come by since being left out of the United squad for last season's Champions League final defeat to Barcelona.
And, on the eve of a last-16 Europa League second leg tie with Athletic Bilbao, Ferguson confirmed the Bulgarian is set to leave in the summer.
"His concern is not getting first-team football," said Ferguson. "At 31, he is looking at first-team football.
"It is difficult for me to guarantee that so it could lead to him looking elsewhere."