Injured while away with Brazil this week, the defender may be missing until mid-December.
Speaking to Betfair, Shearer said: "News of Gabriel's injury won't be pleasing for Arsenal fans, but they have got the squad depth to come in. They have other options and it's not as if they're down to the bare bones at Arsenal. They've got the biggest squad they've had for a while in terms of size and quality.
"Gabriel is a top defender and I don't care who comes in, it will be tough to take. Arsenal have good players but there aren't many Gabriel's around. That's why he gets the reviews he does because of how good he is.
"Defensively he's rock solid and has played a huge part in what Arsenal are doing but certainly from set-pieces, he's a huge cog in what they do in terms of scoring goals and making it very difficult for defences as well. We know how good Arsenal are with set-pieces and he plays a major part in that.
"Although Arsenal have a good replacement to come in and replace him, they're losing probably the best centre-half in the Premier League."