The Canadian club kicked off 2012 in disastrous fashion, losing their first nine matches under Aron Winter, but rallied when Mariner was appointed in June.
Toronto won for of their next nine and it looked like things were back on track for an ok campaign until Dutchman Koevermans, who had scored nine goals in 16 outings, went down with a knee injury. After that, it continued to go wrong for the Reds as they ended up with just five wins for the season and Mariner feels it was that injury which cost the club.
"The turning point was Koevermans going down. It was a big blow for everybody, obviously for Dan," he told MLSsoccer.com.
"A big blow for me because I thought we were working together very, very well. His leadership qualities off the field and on the field were at their highest point, and it sort of took the wind out of our sails a little bit.
"Goals are a premium in Major League Soccer. And you have somebody [in Koevermans] who you feel may score in every game.
"We were creating some very good chances, we were supplying him well. What you do as a coach, you hope somebody can come in and fill those rather large shoes."