Ronny Deila cut a frustrated figure as his Celtic side were held to a 1-1 draw at Hamilton.
Northern Ireland keeper Michael McGovern saved a second Leigh Griffiths spot-kick and the Hoops, who played the majority of the game with 10 men, squandered several chances.
Deila said: "I'm disappointed that we went down to 10 men as we were in control.
"In the second half we should have scored two and killed the game. We didn't, they got a chance and finished the set-play.
"We can be braver on the ball and it's still so close when you get two alone with the goalkeeper and miss a penalty.
Deila also praised McGovern, saying: "It's a good save but we should have finished off our chances."
Griffiths added: "I thought Michael was going to go the same way I went the first time. That's twice he's saved a penalty from me.
"I take responsibility. I scored the first and was confident for the second. Credit to Michael, he made the save."
A superb strike by substitute Eamonn Brophy earned Hamilton the draw against the Ladbrokes Premiership leaders.
Griffiths scored from the spot in the 35th minute after being brought down by Dougie Imrie before Parkhead defender Dedryck Boyata was shown a red card by referee Craig Thomson for a last man tackle on Carlton Morris just outside the box.
In the 65th minute, Accies' skipper McGovern saved a Griffiths penalty after Thomson had penalised defender Ziggy Gordon for wrestling with Celtic counterpart Mikael Lustig at a corner.
Brophy - who had scored in the 8-1 defeat at Parkhead in January - came on for Darren Lyon in the 70th minute and levelled four minutes later with a drive from inside the box for a share of the spoils which took Celtic seven points clear of Aberdeen, who host St Johnstone today.
On the dismissal, Deila said: "I think it was an OK decision. I saw it one more time and I don't think he touched the ball.
"Rule number one - stay on your feet and run with people. That's something we have to work on."