The defender helped United to a 4-0 victory for Chelsea in his first game for the club, with his display earning him plenty of praise from pundits afterwards.
Maguire picked up the man of the match award after helping to keep the Blues at bay, but Nicholas was left unimpressed with his performance.
“I thought Harry Maguire was average – he did everything that was asked of him,” Nicholas told Sky Sports.
“He was efficient and got about the pitch, but I don’t want to hear nonsense about him being a leader and strong on the pitch already – it is too soon.
“He has started well, but I am suspect about the full-back positions at Man Utd. Chelsea could have easily scored two or three.
“I am still not convinced by United and I would still sell Paul Pogba.”
Maguire will make his second start for United at Wolverhampton Wanderers on Monday.