He was later brought around but Strachan admits the situation was concerning at the interval.
"That was a problem at half-time, when he was in the dressing room and we were worried about Stephen, really concerned," Gordon Strachan told Setanta.
"The fact is that he's regained consciousness, so I was going to pop in and see him, but it's a waste of time just now because he'll be having a lot of tests at the moment."
McManus now seems set to miss Wednesday's Champions League clash with Benfica.
Strachan added: "We keep our fingers crossed that Stephen's going to be fine."
McManus, 25, is also a key man for the Scotland team, whose next match is the crucial Euro 2008 qualifier against Italy on November 17.
Scotland manager Alex McLeish said: "I certainly hope that he's fit for the national team next month."