Not to the degree this guy is suggesting, absolutely not. He was in and out of the team last season and the main reasons for this season's demise are Vinnie, laporte, some uncharacteristic ederson mistakes, fernandinho's positional switch and Bernardo's loss of form following twitter-gate. I don't care what anyone says, Mahrez has been absolutely brilliant this season and we are creating every bit as many chances as previous years but we're missing them (sterling against Chelsea prime example). And in fact we're the league's top scorers & still have a good chance of outscoring last season's tally.
So, no I don't think it's been a "major" factor. It's the other end of the pitch that's let us down.