Chilwell suffered the injury in the dying embers of Chelsea's 2-1 win over their Croatian counterparts but their celebrations for topping their group were curtailed by the late injury suffered by their left back, who had to be helped off the field by the club's medical team.
Raheem Sterling and debutant Denis Zakaria scored for the hosts after the visitors took an early lead.
Potter acknowledged afterwards that Chilwell's injury does not look good.
"Not great, he pulled up like that when he turned – not positive at the moment," Potter said of Chilwell's injury after the 2-1 Champions League win.
"It's a blow for us after an otherwise positive game, to see him pull up like that is not a great sight.
"Fingers crossed when we get it scanned it isn't as bad, it can be not as severe, but clearly we are disappointed."
England manager Gareth Southgate is set to name his 26-man squad for the World Cup in less than a week and will be sweating on the fitness of another of his full backs.
Reece James is also a doubt for the tournament and Kyle Walker is still working his way back to full fitness.
Chilwell has won 17 caps for England and was part of the Three Lions squad at last summer's European championships, although he did not play a single game in their run up to the final.
N'golo Kante is another Chelsea player who has been ruled out entirely of the World Cup through injury.
