Grealish is currently part of our best XI. But the best XI is not a permanent thing, it's dynamic, players lose and regain form. Cancelo and Rodri were part of our best XI when we won 21 games in a row, but lost form toward the end of last season and didn't play in the CL final. The season is long, at different times we'll have a different best XI. Remember that Gundo is 31 and Kevin is 30. They will be rotated. Also, some players will get injuries. Again, the best XI is a dynamic thing. The likes of Stones, Zinch, Mahrez, Torres and Sterling may hit top form at some moment and become part of the best XI. Nobody expected that Gundo was going to be one of our most important players last season.
How is Grealish part of our best XI? He can currently only play on the left and he is not going to displace Foden