Injured Ibra ruled out for at least 10 days

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WATCH: Zlatan brace returns AC Milan to Serie A summit 

Ibrahimovic, 39, kept Milan top of Serie A with a brace in a 3-1 win at Napoli on Monday (AEDT) but left the field shortly before the end of play at the Stadio San Paolo with a left thigh knock.

The source told AFP that tests on Monday morning revealed damage to the hamstring muscle in Ibrahimovic's left thigh, and that the Serie A top scorer would undergo new tests in 10 days' time.

As well as missing Thursday's Group H match at Lille, the former Inter Milan, Juventus, Barcelona and Paris Saint-Germain forward will also be unavailable for Milan's next league match with Fiorentina and the visit of Celtic early next month.

The Swede's absence is a big blow to Milan coach Stefano Pioli, whose team have been galvanised by their ageing frontman since his return to the seven-time European champions in January.

Milan is top of Serie A and unbeaten in domestic competition so far this season, with Ibrahimovic scoring 10 of its 19 league goals.

The 'Rossoneri' are also second in Group H heading into Thursday's match, one point behind Lille which is the only team to have beaten Milan this campaign after romping to a 3-0 win at the San Siro before the international break.