HECTOR BELLERIN and Kieran Tierney have handed Arsenal a major boost ahead of Sunday's trip to Watford after the duo took part in full training on Thursday.  While Rob Holding is ready for selection having played for the U23 side in recent weeks in order to get back to match fitness.  Spanish right-back Bellerin has been out of action since January after he ruptured his knee ligaments in the 2-0 win against Chelsea.  While Tierney is yet to make his Gunners debut having joined from Celtic in the summer for a fee in the region of £25million.  The 22-year-old left-back has been hampered by both knee and groin injuries, while he also underwent a double hernia operation in May.  Neither player are expected to be involved against the Hornets, though they could both get some game time at the end of September.  The duo could play some part in the Carabao Cup tie against Nottingham Forest on September 24 as well as the Europa League clash against Standard Liege on October 3.  Unai Emery could decide to rest Granit Xhaka on Sunday after it was revealed he has been playing with an Achilles injury since the start of the season.  And Lucas Torreira was a doubt to face Watford at Vicarage Road after he picked up a knock while on international duty with Uruguay.  But a statement from Arsenal on Wednesday declared no player suffered setbacks while away with their countries over the past week. Arsenal hero Per Mertesacker admits he would get ‘p****d’ off at Mesut Ozil over his attitude  They said: "All players who were on international duty will have returned to us by Thursday morning.  “No international players have reported injuries during this period.  “However, all international players will be assessed ahead of Sunday’s match.”