Gordon trained with Newcastle in the morning, but was named on the bench before kickoff due to illness.
The attacker eventually came on in the 67th minute as Newcastle and Barca played out a 1-1 draw in the round of 16 first-leg.
Shearer snapped on Amazon Prime: “Maybe I’m old school but if you’re fit enough to train in the morning, and I know he doesn’t feel well, but this is Barcelona at Newcastle for a place in the quarter-finals of the Champions League.
“It would take something extraordinary to keep myself out of this game tonight.
“And then to come and say ‘I might be able to give you half an hour at the end’, I don’t know, give us half an hour at the beginning and if you’re feeling alright, you get through to half-time.
“With his pace and his form in the Champions League he’s going to be a huge miss, so that surprises me a lot.”