Aberdeen keeper Dimitar Mitov will be sidelined for six weeks after suffering a hamstring injury.
Bulgarian international Mitov was replaced at half-time during the 1-1 draw at Hearts on Sunday due to injury.
A scan has revealed Mitov will be out for the rest of the year.
Aberdeen have recalled keeper Tom Ritchie from a loan spell at Bonnyrigg Rose as back-up to Ross Doohan who replaced Mitov at Tynecastle.
The loss of Mitov is a maor blow ahead of the top-of-the-table clash against Celtic at Pittodrie on Wednesday.
Thelin is remaining positive and has backed replacement keeper Doohan to rise to the challenge against the Premiership leaders.
The Dons boss said: “That’s football sometimes – players get injured.
“Of course, you want everyone always to be available, but it happens.
“Dimi has been really good, he’s a really good goalkeeper.
“We have a squad, so you have to find solutions and stay positive.
“Now you have to think about the player, to support him.
“It’s always difficult for players when these things happen, but we will support him on his journey back.
“Dimi is a positive guy, we have a good medical team here, so he will be supported.”
Thelin ‘confident’ Doohan can do job
Mitov has been first-choice keeper under Thelin since signing from St Johnstone in the summer transfer window.
Aberdeen triggered a release clause to secure Mitov until summer 2027 for an undisclosed six-figure sum.
The Dons have the option to extend the Bulgarian’s contract to a fourth year.
Former Dundee United and Ross County keeper Doohan, 26, came on at half-time at Tynecastle for only his second appearance this season.
Doohan’s other appearance came when he started the 4-0 League Cup win at Lowland League side East Kilbride in July.
Thelin said: “Ross came in and did well for us.
“He has trained well and we’re confident he will do a good job.”
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