The former Celtic star feels there must be something else behind the striker's dip in form.
Guidetti, on loan at Celtic from Manchester City, began the season in tremendous fashion for Ronny Deila's side but has suffered a remarkable dip in form since.
The 22-year-old's contract expires with City this summer and he has been in discussions with the Bhoys over a permanent move to the Scottish Premiership.
With the Swede's current loss of form being taken into account, however, Deila has dropped Guidetti down the Celtic pecking order up front.
And McAvennie feels the forward must be carrying a knock - as his dip in performance levels has been too great.
"It looks to me he may have a wee knock that we don’t know about. Take it from me, you just don’t go from being a super-confident striker scoring 11 in 10 games to not scoring at all," he said.
‘He was flying but almost overnight it’s all changed. That’s why I think he maybe has a knock. Unless you’re 100 per cent fit as a striker, you’re maybe going to lack that little extra half-yard of sharpness to get in front of a defender and score goals."
Such is the strength in depth of Celtic's squad at the moment, Guidetti's absence has not been too greatly felt up front. Leigh Griffiths has stepped up to the plate, while Mikael Lustig, Liam Henderson, Adam Matthews and Virgil van Dijk have all also scored recently.
All of their goals have seen Celtic return to the summit of the Scottish Premiership table, while Aberdeen have drawn their last two games in a row to fall slightly off the pace.