The former footballer said that Lukaku, who has only netted twice for Jose Mourinho's side in the last 14 outings, is not proving worthy of the £75m price tag which brought him to Old Trafford in the summer.
"He looks to be lacking in confidence, two in 14 now but in his defence with some of the balls and some of the service he was feeding on scraps," Shearer told Match of the Day 2.
"Everything seems a second or two too slow and by the time he's made his mind up it's gone. He's not doing enough in games - and I hate saying it - to justify his fee.
"No one was asking these questions earlier on in the season when Lukaku was scoring 11 goals in his first 10 games. No one was asking the question of Mourinho then and whether he had improved Lukaku."
The Belgian international, whose side face Bournemouth on Wednesday, has only scored twice in 40 games against the Premier League's so-called top teams.