Cheers mate figured it out. He has it recorded as 19 OPTA, he just hasn't added the Rayo game yet.
So that leaves 1 less which was his assists to Suarez against Altetico.
http://www.lastminutegoals.org/barcelona-vs-atletico-madrid-la-liga/
Take a look at it closely, it clearly deflects off the defender. It is record on the list as (d) for deflection. According to OPTA its not an assists so Messi has 19 assists this season, not 20. This is where the world of assists becomes a bit contentious.
Now we just have to find the other 4!
Thanks I found what I was looking for, whoscored gave Messi the assist to Suarez against Atletico which appears to be incorrect.