Jackson had been an automatic starter for Chelsea until his red card at St James's Park in April - his second dismissal of the season - which left the club without a striker as they were competing for a place in the Champions League.
He was then sent off within four minutes of coming on as a second-half substitute in the 3-1 group-stage defeat by Flamengo at the Club World Cup, Fifa's expanded competition which Chelsea ultimately won. Strikers Delap and Joao Pedro arrived to replace him during the tournament.
Multiple sources with knowledge of the situation said Jackson ultimately lost the trust of manager Enzo Maresca and some of the hierarchy at Stamford Bridge, prompting him to seek a move.
It was a shift in mood as Maresca said earlier in April that the frontman was his "perfect number nine" and wanted "a copy of Nico" rather than to replace him.
Jackson could face Chelsea on September 17 (20:00 BST) in Munich as Uefa rules allow loanees to face their parent clubs in the Champions League.