The 22-year-old has since helped Saracens to the top of the Premiership and enjoyed a successful spell in Stuart Lancaster's England team, as they beat Ireland, Wales, Scotland and Italy over the last two months.
McCall is now hoping Farrell can steer his side to domestic and European success before the end of the season - and the Lions tour has put him in the best possible position to do so.
"He can get better, of course he can," said McCall. "You can all see how much he's improved since last summer.
"I think the Lions tour has made a huge difference to him. He's in great form at the moment and is such a mature 22-year-old.
"We rotate the squad regularly throughout the season, so Owen's game time is exactly what it should be at the moment - just using him as an example."