Hot shot Donaldson played a key role by bagging 18 goals as the Bees clinched promotion out of Sky Bet League One in the 2013/14 season as runners-up to Wolverhampton Wanderers.
The Bradford-born player began his professional career with Hull City, before moving on to York City, Hibernian, Crewe Alexandra and then Brentford in 2011.
Following his switch to the Bees, he had a better than one in three goal average, having found the back of the net 53 times in 155 appearances.
Donaldson made his Blues debut as a second-half substitute in the opening-day defeat at Middlesbrough.
His first goal in royal blue was against Cambridge United in the Capital One Cup tie, which Blues won 3-1.
He ended his first campaign as the Club's top scorer with 16 goals in all competitions.
His efforts saw him rewarded with both Players' Player and Supporters' Player of the Season awards.