Man United got back to winning ways with a 1-0 victory in the Europa League clash, thanks to Anthony Martial’s penalty.
Martial played one hour on his return to the starting line-up, before being replaced by Marcus Rashford.
Rashford failed to add more goals, with United failing to score more than once since August.
Ahead of their trip to Norwich this Sunday, Solskjaer told MUTV: “Second half I thought we dug in really well in the box, defended really well in the box. They played well in the second half and made us defend too deep.
“I’m not happy with the defending at the front. I think that’s been one of our strengths this season but I thought that was non-existent in the second half.
“That needs sorting out because we don’t want to be a team who is just defending in the box.”