Pep Guardiola says he feels responsible for not being able to “drive” Manchester City beyond last season’s Champions League quarter-finals.
City, who have never won European football’s top club prize, begin their 2020-21 campaign at home to Porto in Group C on Wednesday (20:00 BST).
They lost 3-1 to Lyon in a one-legged last-eight tie in 2019-20.
“Still, right now, I feel responsible watching the game, so we have to accept the reality,” said City boss Guardiola.
Since Guardiola was appointed in 2016, City have failed to advance beyond the quarter-finals, reaching that stage three times.
Having knocked out Real Madrid in the last 16, they were strong favourites to reach last season’s final, but failed to get past French side Lyon in a ‘final eight’ knockout tournament played in Lisbon amid the coronavirus pandemic.
“It was a tough moment,” added Guardiola, who won the Champions League twice as Barcelona manager.
“I felt so responsible for this: how the club and players fought to achieve it and I was not able to drive them.
“The reality is we have not performed to the level we have to to get through in this competition. We have to accept it with humility, accept all the bad situations that we have.
“It is the best way to improve and go through and accept a new challenge this season. Start from zero again and try it again, knowing we have to improve.”
Defenders Aymeric Laporte and Benjamin Mendy, midfielder Kevin de Bruyne and forward Gabriel Jesus will all be absent for Manchester City on Wednesday, although left-back Olek Zinchenko is available.
“We expect Kevin and Aymeric to return in the next few weeks,” said Guardiola. “Gabriel, maybe a little bit longer and we hope they will come back soon and the team will grow.”
Having been crowned champions of Portugal last season, Porto are second in the table, five points behind leaders Benfica, four games into the 2020-21 campaign.
Earlier this month, they signed West Ham’s Felipe Anderson, Liverpool’s Marko Grujic and Chelsea’s Malang Sarr on season-long loan deals.
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Manchester City have won both of their previous two meetings with Porto, winning 2-1 away and 4-0 at home in the Europa League in 2011-12.
Since his Champions League debut for City in September 2015, forward Raheem Sterling has scored 20 goals in the competition – the joint-most of any English player, along with Harry Kane.
The last team to beat Manchester City in the group stage of the Champions League was Lyon, in September 2018.
Porto have lost just two of their past 16 Champions League group matches, winning 11.
This will be Porto’s 21st away game against English opponents in European competition. They have drawn three but are still looking for their first victory.
The Portuguese champions have not won their opening group match in any of their past four Champions League campaigns.