Wales, who were beaten in their Six Nations opener against Ireland at the weekend, have lost their last eight matches, five under the guidance of Howley. However, ahead of the weekend's trip to face France in Paris, Roberts has moved to back the coach.
"We are 100% behind Rob Howley, without a doubt. In comparisons with the standards we have set as a team, that first half-hour was very disappointing and it cost us the game. It is one of the worst Wales performances I have ever been involved in," Roberts told Wales Online.
"Pundits are always going to say negative things but the important thing is we stay tight as a group. Saturday night hurt. I could see it on the faces of all the boys. The fact is, if we defended better in the first 20 minutes we would have won the match. But we were 20 points down before we touched the ball.
"It is hugely frustrating because we all know the structure is there for us to win rugby games but we made the wrong decisions on the ball against Ireland. We all know that where there are chances to score tries there are chances to win games and we know we can cause France problems on Saturday."
France were also beaten in their Six Nations opener against Italy.