Johnstone, 29, played 37 times for West Bromwich Albion in the Premier League in the 2020/21 season, winning both the Supporters’ and Players’ Player of the Year awards, and remained a key part of the side in the Championship during the 2021/22 campaign, at which point his contract came to an end.
He started his career at Manchester United, rising through the Academy and excelling on loan in the Championship, most notably at Aston Villa.