The Norwegian took over the reins at Swansea City's fierce rivals in January this year, but failed to keep the club in the Premier League as they suffered relegation back to the second tier.
Cardiff have struggled to adjust to life back in the Championship, winning two of their opening seven fixtures to lie 17th in the early standings.
In a press conference on Thursday, Monk expressed his regret over what had happened to Solskjaer.
"Cardiff have had a difficult transition," he said. "They had a difficult first season in the Premier League and having a new manager come in just around Christmas is difficult.
"I don't know the ins and outs but Ole was a very good guy and very knowledgeable, I'm disappointed for him.
"He wanted to change the philosophy, maybe play more football, but it never quite worked out.
"I feel sorry for him, from the outside he seemed to have good working relationship with players but it's hard to know the ins and outs."
Swansea were hit by the news that defender Jordi Amat would be out for up to six weeks with a knee injury, and Monk sees the setback as a chance for another player to make a name for himself, with Southampton visiting the Liberty Stadium in Saturday's Premier League clash.
"When someone gets injured it's an opportunity for someone else," he said.
"We've got four good centre-halves. We will see."
