The Scotland international put an end to his six-year association with Celtic in the recently concluded transfer window, joining Championship club Bournemouth.
Christie was among three players who left the Glasgow giants to move to England, with Kristoffer Ajer signing for Brentford and Odsonne Edouard joining Crystal Palace.
Former Scottish Premiership star Foster is of the view that the likes of Ajer and Edouard are hoping to follow in the paths of former Celtic defender Virgil van Dijk and earn a move to a top club.
Foster, though, explained that he cannot understand Christie’s decision to join Championship side Bournemouth while he could have pushed for titles and trophies at Celtic.
“Ajer went to Brentford and they are in the Premiership, but with all respect, it is Brentford“, Foster said on BBC Sportsound.
“Edouard went to Crystal Palace and he has had a great start.
“I know these guys are probably hoping to do what Van Dijk did and bounce on and then get to the top teams.
“But, Christie is the one that I really can’t understand.
“You’ve left a club like Celtic, who are challenging to win titles, win cups.
“If they win the league this year they go straight into the Championship group stages, and he has left to go to Bournemouth.“
Foster suggested that playing in the Premier League is not a major achievement and insisted that Christie should have looked to win more trophies like Brown, who he feels is one of the most decorated Scottish players.
“If I look back at my career, the highlights of my career, I want to have won things, not at what level I played at“, Foster added.
“If you said to me ‘Well done! You’ve played your entire career, you’re tenth in the Premiership every year’ and what?
“You have achieved nothing, you’ve played in the Premiership, you’ve earned vast sums of money, great.
“I know Ryan Christie has won things, but win more.
“Look at Scott Brown, look what he has won, probably the most decorated Scottish player ever.“
Having left Celtic for Bournemouth, Christie will be hoping to help the Cherries earn promotion to the Premier League this season.