The 34-year old St Mirren striker joined Coyle at Burnley from Cardiff City for free back in 2008 and the his ex-boss believes he had a bigger impact on the team than the likes of Jack Wilshere (to Bolton on loan from Arsenal), Daniel Sturridge (loan from Chelsea), Steven Fletcher (signed to Burnley in 2009) and Gary Cahill (signed to Bolton in 2008).
"We had very good attacking players at Burnley but what I didn't have was a big powerful striker to lead the line," Coyle told SunSport after Thompson's goal helped St Mirren beat Celtic in the Scottish League Cup semi-final on Sunday.
"I'd watched Thommo do well at Cardiff but he wasn't starting. They wanted a lot of money which I didn't have. I persuaded their chairman Peter Ridsdale and we got him on a free.
"But because of what Thommo brought to the team he was probably the best signing we made.
"You can have good players but sometimes it's about getting the final bits of the jigsaw. He was the ingredient we were missing and we went on to have an unbelievable spell as he brought so many facets to our play at the top of the pitch.
"Again on Sunday he did it in the big occasion. He thrives on the atmosphere. I've no doubts you'll see a huge performance from him in the final. I was delighted for him though everyone knows I'm Celtic-minded."