Rudi Garcia blamed a lack of balance for Roma's defeat in the Champions League on Tuesday, though he said he thought his side had done enough for a draw.
Two goals from full-back Filip Mladenovic powered Belarusian underdogs BATE Borisov to a surprise 3-2 win.
Captain Igor Stasevich headed in the eighth-minute opener after the ball took some helpful deflections and bounces in front of goal, while Mladenovic's first goal four minutes later was a speculative long-range effort which goalkeeper Wojciech Szczesny seemed to lose sight of.
Roma mounted a comeback in the second half to close the gap but were denied a late equaliser when Alessandro Florenzi's powerful shot bounced back off the bar.
Garcia rued his team's first-half performance after the game.
"Obviously we got the first half wrong," Garcia told Mediaset. "Perhaps we lacked balance, seeing as we conceded all the goals on the same flank.
"When Vasilis Torosidis came on, he balanced things out again. We then paid a heavy price for their clinical finishing and a little luck, as we hit the crossbar with Alessandro Florenzi and it went out, they hit the bar and it bounced in.
"Football is made up of errors and we made more of them. At least in the second half we did everything possible to get back into the game and deserved a point. We didn't deserve to win obviously after that first half, but a draw would've been fair."
Garcia has only three wins in 20 Champions League games in his career, and Roma sit bottom of Group E with one point, trailing Barcelona on four and Bayer Leverkusen and BATE Borisov with three each.
"It was possible to get points here, but we conceded on the first counter-attack and the team then fell apart," Garcia said. We had to put everything back on track in the second half.
"I don't think we took the wrong approach, because the opening five minutes were good, as we created chances and pressed them.
"We paid for their first efforts and when 3-0 down it's more complicated to come home with points. Luck wasn't on our side either, but it is our fault and mine too if we played so badly in the first half.
"Tonight we weren't sharp, also as there were so many players missing. I thought of fielding my best attack, but in the end we lacked balance.
"We need to make fewer errors to be more consistent. Against Sampdoria we also made too many mistakes. There was a reaction, which shows our mentality is right. Now we need to get a result at Leverkusen to qualify.
"We lost many one-on-one duels in the first half and won many in the second, so that is an important sign. We didn't fight enough in the first half and suffered their pressing, more due to our fault than anything else.
"We had the chances to equalise, albeit conceding three goals."