Lukaku has scored eight goals in the Premier League this season but the majority of them came between August and October.
His last goal for United in the league was last month in the 4-1 win over Newcastle.
Patience is starting to run out, according to Shearer, after the Belgium striker’s poor performance in the 2-1 defeat by Manchester City at Old Trafford.
Lukaku was to blame in the build-up to both of City’s goals on Sunday and questions are starting to be asked.
‘No one was asking these questions earlier on in the season when Lukaku was scoring 11 goals in his first 10 games,’ Shearer told BBC’s Match of the Day 2.
Lukaku was expected to be Manchester United’s focal point but fired a blank against City
‘No one was asking the question of Mourinho then and whether he had improved Lukaku. He’s not doing enough in games – and I hate saying it – to justify his fee.
‘He looks to be lacking in confidence, two in 13 now but in his defence with some of the balls and some of the service he was feeding on scraps. Everything seems a second or two too slow and by the time he’s made his mind up it’s gone.’
Shearer also scrutinised Lukaku’s performance in front of goal. He was denied by City goalkeeper Ederson from close range in the second half as United chased the game.
The striker spurned a very good opportunity in the second half as United chased the game
He added: ‘From a centre forward’s point of view that’s what makes the difference between the very good strikers and the good strikers. You get one chance like that and it has to be in the back of the net.
‘His mindset now – I know mine – if i’d missed a chance like that I wouldn’t sleep for the next three or four days until the next game or until i’d scored again. That missed chance would be on my mind until then.’