Romelu Lukaku has been sent for an MRI scan on a foot injury, the Belgium national team have confirmed.
And Belgium confirmed on Twitter that the 25-year-old, who has scored 45 goals in 79 international appearances, has had a scan to determine the extent of the problem.
The Red Devils kick-off Group I with a home match against Russia at the King Baudouin Stadium in Brussels on Thursday, before travelling to face Cyprus in Nicosia on Sunday.
Should Lukaku not be fit, Martinez has Dries Mertens, Christian Benteke, Divock Origi and Michy Batshuayi as alternative striking options.
Lukaku has made a welcome return to form for United since Ole Gunnar Solskjaer replaced Jose Mourinho as manager on December 18.
The striker's goals had dried up under the Portuguese, netting just twice in the club's final 12 games under the previous regime.
But the appointment of the Norwegian has helped revive his form in front of goal, and he has scored four in his last three Premier League outings.
His most crucial contribution has come in Europe, where his two goals in Paris helped set United up for a famous comeback against PSG to haul the team through to the Champions League quarter-finals.
Any extended lay-off for Lukaku will be a blow for Solskjaer, who has seen his team's sensational form under his tenure begin to flag in recent weeks.
United lost back-to-back games for the first time under the caretaker boss after losing 2-0 to Arsenal in the Premier League and crashing out of the FA Cup quarter-finals with a limp 2-1 defeat away at Wolves.