The Celtic boss was full of praise for the 23-year-old.
Celtic won 2-0 to strengthen their grip at the top of the Scottish Premiership after a goal from Forrest and a Stefan Johansen penalty.
But it was the former that drew more of the Norwegian's attention.
Forrest has been much criticised by Celtic fans and former players this season but his manager has continually stated that he has the talent to come good.
After the St Mirren win, the Bhoys boss found himself saying more of the same - only in hindsight.
RD on Forrest: "We have been very patient with James and we have been talking about him a lot this season."— Celtic Football Club (@celticfc) April 3, 2015
RD: "Now we are seeing what we can get out of that. Today he was sharp and had a hard fight against the full-back but he didn't give up."— Celtic Football Club (@celticfc) April 3, 2015
RD: "He kept going and in the end he won the game for us so he’s a player who’s very important for us."— Celtic Football Club (@celticfc) April 3, 2015
Has patience been the right approach and has Forrest come good?
He certainly did his bit on Friday night - and the Scottish Premiership table now makes for superb reading for Celtic fans.
Could a first domestic treble since 2001 be on the cards for the Hoops? More wins and more performances like that at St Mirren certainly suggest so.
And Forrest could have a bright future indeed at Celtic, although the 23-year-old needs to prove his worth of a long period of time, of course...