When asked if Liverpool would stick or twist with their manager at the end of the season, Castles said: "Liverpool will twist if Klopp fails to get Champions League football or win a trophy this season - which is looking a distinct possibility".
"He was in a good way and then had a little setback".
The Scotland worldwide was considered the answer to the club's problems at left-back, and put in a man-of-the-match performance against Crystal Palace on his debut in August.
Klopp also revealed that Liverpool had shown the England international's back problems to multiple specialists all around the world in an attempt to find a solution to his long-term absence.
Clyne reportedly travelled to London with a member of the club's medical staff for corrective surgery on Monday.
Klopp will be undoubtedly be left frustrated should Coutinho pick further hinder his rehabilitation during the clash with Gareth Southgate's England, with Southampton full back Ryan Bertrand - Liverpool's next opponents - among a depleted Three Lions outfit.
The Scotland defender has played more games for his country than for the Anfield club this season.
Details of Clyne's injury did not emerge until he was omitted from the Reds' Champions League squad in late August - a move that meant Liverpool were relying on Joe Gomez and Trent Alexander-Arnold for half of the season, with the two youngsters both being inexperienced as the season began.
"Obviously it is a concern that the hamstring injury is again on the agenda and it will be critical for us to monitor and manage that in the coming days", Klopp told liverpoolfc.com.
Robbo said: "I've spoken to the manager because I felt as if the settling-in period was over for me and I thought I was ready to play".
Monday saw a return to training for all the players who are not away on worldwide duty.