Garnacho has so far struggled in west London, registering just six goals, with just one of those coming in the Premier League under managers Enzo Maresca and Liam Rosenior.
The Argentina international has failed to impress so far for the Blues and many are worried that he will get back his best under Rosenior who has played him off the bench since he took over from Maresca.
Speaking to NewBettingSites.uk, Nevin admitted he is concerned and revealed many are confused as to why the club snapped him up.
"I was at the West Ham game and only about the 252nd person came up to me and said the same phrase... 'Why are you getting rid of Noni Madueke and bringing in Alejandro Garnacho?
"I felt sorry for Garnacho. They had to take him off. It was actually a wee bit sad. The last time that happened at Stamford Bridge it was Raheem Sterling. I’m talking seriously now, as a footballer when that can happen, if it takes a turn and the confidence goes completely, it can become very ugly.
"Garnacho needs to be very carefully managed but remember he scored two goals against Arsenal a couple of weeks ago. There have been moments where it’s been great but he’s not won those Chelsea fans over yet."
Garnacho let Old Trafford after being placed in Ruben Amorim's 'bomb squad' last summer and could be on the transfer list once more in the summer if he does improve his poor form as the club seek to construct a title contending side.