The burly Belgian was shipped off to Inter Milan for £74m, with United failing to sign a replacement.
And Shearer thinks the decision could have negative ramifications for Ole Gunnar Solskjaer.
"I get the feeling Marcus feels more confident coming in off the left where he does not have to play a lot with his back to goal.
"There is no denying he is an excellent talent but I don't think he is a natural goalscorer. We sometimes forget he is only 21 so time is still on his side.
"But he is very different to Harry Kane, who is determined and ruthless. I do not think Marcus will be bothered by the England boss' words.
"We have not heard from Marcus where he prefers to play and I have never spoken to him about it.
"But Gareth probably would not have said that if Marcus had told him he sees himself as a centre-forward."