Paris - Andre Villas-Boas said losing his first game in charge of Marseille on Saturday was "difficult to accept" as he made his return to football.
Villas-Boas' side slumped to a 2-0 home defeat by Reims in their Ligue 1 opener.
The former Porto, Chelsea and Tottenham manager is back coaching for the first time since an acrimonious departure from Shanghai SIPG in 2017, but saw second-half goals from Boulaye Dia and Suk Hyun-jun sink Marseille at the Velodrome.
"It's difficult to accept. We were too slow in attack. We failed to find space," he said.
"Obviously we need to become more confident, and doing so is always more difficult after a home defeat, but we have to keep working on it."
Villas-Boas has been given the task of returning OM to the Champions League for the first time since 2013, after taking over the reins from Rudi Garcia.
But on Saturday's evidence he faces an extremely difficult job with a team who finished fifth last season to miss out on European qualification altogether.
The 41-year-old named close-season arrivals Dario Benedetto and Alvaro Gonzalez on the bench, with France attacker Florian Thauvin missing through injury.
The Portuguese coach saw his team make a laboured start, failing to manage a single shot on target in the first half, with goalkeeper Steve Mandanda forced into fine saves to deny Moussa Doumbia and Dia.
The hosts finally called visiting 'keeper Predrag Rajkovic into action seven minutes after the restart, with the Serbian collecting Dimitri Payet's tame effort.
Marseille midfielder Kevin Strootman rattled the crossbar in the 57th minute, but Reims deservedly found a breakthrough shortly afterwards as Dia played a neat one-two with Tristan Dingome and drilled the ball over the advancing Mandanda.
Argentine international striker Benedetto was handed a Ligue 1 debut with 17 minutes remaining, but failed to make an impact as Reims substitute Suk instead ran clear at the other end to wrap up the three points in the final minute of normal time.
On Friday, Cesc Fabregas was controversially sent off after a VAR review as Monaco were brushed aside 3-0 by an impressive Lyon in the opening game of the season.
Reigning champions Paris Saint-Germain have left out the world's most expensive player Neymar for their first match against Nimes on Sunday, admitting talks over a possible move away for the Brazilian are "more advanced".