The midfielder was playing in the unfamiliar position of right-back in the absence of three other players, Mark Wilson, Jean-Joel Perrier Doumbe and Paul Caddis.
The Scotland midfielder was substituted after 18 minutes of the win, courtesy of a late strike by Scott McDonald, that takes them back top of the Premier League, two points above Hibernian.
Strachan broke the bad news for Alex McLeish, saying: “Paul has run about forever this season but 15 minutes in that right-back position and he pulls a hamstring. That is usually a three-week job.
“Gary Caldwell and Stephen McManus picked up knocks but they won’t be pulling out.”
Another late strike by McDonald gave Celtic three points in a dramatic afternoon in Lanarkshire.
Fabian Yantorno’s 30-yard free-kick after 36 minutes gave the Border side a shock half-time lead and for most of the second half they answered all of the questions asked by the visitors.
However, the last five minutes left them pointless once again and still bottom of the table.
Chris Killen headed Celtic level in the 85th minute with his first goal for the club then McDonald, who had fired in a late winner against AC Milan in midweek, drove home from 12 yards out after an Aiden McGeady shot had landed at his feet.
Strachan claimed his side needed a “reality check” after a poor first half. He said: “In the first half the surface and the ball was not conducive to good football. Plus the way that they were playing, they were strong and getting that goal gave them extra power and strength.
“So I told the players to play reality football.
“It is in the players’ mindset to play entertaining football but it wasn’t working, it was horrendous to watch so we needed to get back to reality.
We were fortunate to get a win but I thought it was likely that we would get a goal at the end.”
Gretna defender Evan Horwood, whose header off the line when the score was 1-0 to Gretna foiled McDonald, apologised to his team-mates for playing the Australian striker onside for the winner.
He said: “I stayed in and I cost the side the game. I apologised to the players for that but I couldn’t have done anything, I was tired because I had worked my socks off.
“The stats will say they tortured us but they didn’t deserve either goal.
“I saved one off the line but I cost us the game.
“We stuck to the game plan and we did well, it was the last five minutes that killed us.”
Gretna boss Davie Irons could hardly believe that his side had lost the game.
He said: “To say I am disappointed is an understatement. I thought we had something from the game. The boys were magnificent and their effort was clear for everyone to see.”
Fleming 6, Barr 6, Innes 6, Cowan 7, Collin 6 (Grainger 14, 5), Horwood 6, Jenkins 5, Murray 6, Skelton 7, Yantorno 7 (Paartalu 81, 5), Grady 6 (McMenamin 64, 5). Subs Not Used: Caig, Graham, McGill, Hogg.
Boruc 5, Hartley 6 (Kennedy 18, 5 (O’Brien 75, 5)), Caldwell, 6, McManus 6, Naylor 6, Nakamura 6, Scott Brown 5, Donati 6 (Jarosik 80, 5), McGeady 6, Killen 7, McDonald 8. Subs Not Used: Mark Brown, O’Dea, Sno, Riordan.