Sebastian Rudy and Zdravko Kuzmanovic gave Stuttgart victory at Ibrox last night.
“We are obviously disappointed,” he told Sky Sports.
“We have lost our three home games in the Champions League and it has obviously been difficult for us.
“When we are at home the onus is on us to go forward and we haven’t handled the situation as well as we have done in the away games.
“In all the home games we have had opportunities to take the lead and haven’t managed to take them.
“But the bottom line is we haven’t been good enough in the home matches. It’s strange because we have played well away from home, but have never reached that level at home, which is difficult.
“If we play three games at home and don’t take a point, we don’t deserve it. It’s as simple as that.”