The stop-start season for Ajax Amsterdam and Bafana Bafana playmaker Thulani Serero hit another hurdle this week when he was injured again and will miss Thursday night's Uefa Europa League clash against Glasgow Celtic. It is a huge frustration for the 25-year-old, who had only recently returned to the starting 11 after missing most of Ajax's pre-season in the treatment room.He has been replaced by Donny van der Beek.Serero started the past weekend's 5-1 thumping of Cambuur in the Eredivisie, but was replaced seven minutes into the second half with more ankle trouble.That was just a second start of the campaign for Serero, who earlier in the month featured in both Bafana's World Cup qualification matches against Angola, one from the start and the second off the bench.The length of his absence should be determined in the coming days, but it looks unlikely that he will feature in Sunday's tricky away league clash at PEC Zwolle.Ajax could be knocked out of Europa League contention if they do not win in Scotland, which would be a blow to South Africa's lastremaining player in European club competition this season.Norwegian side Molde are the surprise leaders of the pool with 10 points from four games, followed by Turkish giants Feyenoord (five), Ajax (three) and Celtic (two).No South African has ever won the Europa League competition, formerly the Uefa Cup.Spanish La Liga side Sevilla are the current holders...