Tiago is likely to be sidelined for at least two weeks after medical tests confirmed that the Atletico Madrid midfielder has sustained a hamstring injury.
The 35-year-old replaced Nico Gaitan in the 61st minute of Tuesday's 1-0 Champions League win over PSV, but pulled up in discomfort towards the end of the match.
Atleti sent the player for scans that revealed he has sustained grade I-II damage of his right hamstring - an injury that is likely to keep him out of action for close to a fortnight, according to reports in Spain.
"Tiago left the Champions League game against PSV with a problem," a club statement read.
"During the day on Wednesday, the Portuguese midfielder had an MRI scan at the Fremap de Majadahonda clinic, and we now know the exact extent of his injury.
"The medical report shows that Tiago has suffered a grade I-II muscular injury in the femoral biceps of his right thigh.
"The midfielder will carry out separate work in the gym and with our physios in order to return to the team as soon as possible."
Tiago is expected to miss LaLiga matches against Barcelona and Deportivo La Coruna, but Atleti will hope to have him available for the visit of Bayern Munich on September 28.
The former Chelsea and Juventus midfielder has made three appearances so far this season, having missed the majority of last term due to a broken leg suffered in a league game against Espanyol last November.