Ireland international Jonny Hayes appeared to suffer an injury setback in Celtic's friendly defeat to Czech side Bohemians Prague on their pre-season tour of Austria yesterday.
Dubliner Hayes appeared to go off injured during the loss.
Brendan Rodgers' Scottish champions returned to action on Wednesday as they beat SK Vorwarts Steyr 1-0 in another friendly, but this time out they were on the wrong end of the same scoreline.
The result will ultimately mean little, but it was still a defeat as Masek beat Craig Gordon with the help of a deflection 35 minutes in.
Several of Rodgers' key lieutenants were given valuable minutes. Celtic face Sparta Prague tomorrow and then Shamrock Rovers next Saturday before their Champions League qualifier against Alashkert on July 10.
Livingston have appointed Kenny Miller as their player-manager on a reported two-year contract. The 38-year-old former Scotland striker succeeds David Hopkin, who guided them to the Scottish Premiership.