Chelsea qualified for the Champions League knockout stage despite defeat against Werder Bremen - but have injury concerns over top scorer Didier Drogba.
Per Mertesacker celebrates his winner for Werder Bremen
Per Mertesacker's powerful 26th minute header kept Bremen's hopes of finishing ahead of Barcelona in Group A alive.
But Chelsea's delight was tempered by the sight of Drogba limping off with an ankle injury early in the second half.
He will now face a race against time to be fit for Sunday's Premiership clash with leaders Manchester United.
Chelsea recalled teenager Mikel Jon Obi for the suspended Frank Lampard, with Andriy Shevchenko kept in reserve on the bench.
Obi wasted the perfect opportunity to put Chelsea ahead after eight minutes, heading over an open goal after John Terry nodded Michael Essien's free-kick into his path.
Bremen still harbour hopes of overhauling Barcelona to reach the knockout phase, and talented midfield man Diego tested Carlo Cudicini with a right-foot shot after 13 minutes.
Diego then slipped an astute pass through to Hugo Almeida, but he shot wide under pressure from Geremi.
But there was no reprieve for Chelsea after 26 minutes when slack marking allowed Mertesacker to power a header beyond Cudicini.
Chelsea's Champions League ambitions were not going to be derailed by defeat, but they responded in typical fashion.
Joe Cole's cross was met by Michael Ballack, but he was denied a goal on his return to Germany by a fine save from Tim Wiese.
Cole then turned inside the area four minutes before half-time, but shot straight at Wiese.
Chelsea suffered a setback 13 minutes after the interval when Drogba, who had already taken a knock on the ankle, fell awkwardly and limped off.
Shevchenko came on as part of a double substitution, with Arjen Robben also replacing the disappointing Obi.
The £30m striker was quickly in the action, testing Wiese with a 25-yard free-kick.
Chelsea had another anxious moment when Essien fell to the floor clutching his face after a collision with Pierre Wome, but he was able to continue.
Mourinho's side were on top after the break, but were unable to make the breakthrough.
And captain Terry's frustration was demonstrated when he was booked for dissent - but Chelsea will hardly be unhappy at their hugely influential captain missing the final game against Levski Sofia.
The home crowd's nerves were on edge as the closed in on a vital victory, and Essien almost delivered the nightmare finish for Bremen with a stunning rising drive that was only inches off target.
Bremen now just need a draw in Barcelona in their final group game to go through to the last 16.
Chelsea have qualified as they cannot finish below Bremen in the table because they have the better head-to-head record in the group games between the two sides.