Defeat by Hearts should serve as a wake-up call to Celtic's board, says former Scotland player Michael Stewart.
Brendan Rodgers' side struggled to create chances in Saturday's 1-0 loss at Tynecastle.
Stewart believes Celtic's "well has dried up" and they need new signings.
"If this result doesn't tell the board what Rodgers has probably been screaming at them for months - that this team needs freshened up - nothing will," said ex-Hearts player Stewart.
"In the changing room, when there are no fresh faces coming through the door, when you keep looking at the same players to produce a moment of magic and they're not doing it, a wee seed of doubt starts to build.
"All of this starts to take the shine off Celtic - the fear factor starts to go. You better believe the pressure is going to be ramped up in the next few days."
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Celtic, who have won successive domestic trebles under Rodgers, failed to sign long-time target John McGinn this summer, with the 23-year-old Scotland midfielder joining Aston Villa from Hibernian following several rejected bids from the Scottish champions.
Eboue Kouassi, who has made fewer than 20 appearances since joining Celtic from Krasnodar in 2017, started in midfield against Hearts, along with captain Scott Brown and Callum McGregor, and was replaced by Tom Rogic in the second half.
"Celtic paid about £3m for Kouassi," added Stewart. "Look at what McGinn would have brought, not just immediately, but four, five years you'd have got out of him.
"Longer term, he is the natural successor to Brown - no question. That is why that was a symbolic transfer that has fallen through.
"It's staggering that the board dallied on that deal and never got it done."
Michael Stewart was speaking on BBC Radio Scotland's Sportsound