Roberts, 24, came through the match unharmed but was unable to help his side as they crashed out of Europe.
"It was good to be back on the pitch on a personal note but it's a bittersweet moment'" said Roberts.
"It's a very disappointing result. We knew it was the most important game of our season but we have lost at home and it's not a good feeling. It's a disappointed changing room.
"But credit to Northampton. They played the right sort of rugby when we went down to 14 men, were dominant in the scrum and also threatened with their back three."
Roberts is now hoping the Blues can defend the Amlin Challenge they won last year after failing to advance from their Heineken Cup pool.
"We have a chance to try and repeat what we did last year and reach the quarter-finals of the Amlin. That's our aim now," continued Roberts.
"We need to win our last two European games and also pick up bonus points along the way as there are teams in other pools with more points than us now."