The Wales midfielder lasted just 16 minutes against Hull City in the fifth round replay.
The 26-year-old forward admitted the meeting took place without Arsene Wenger's knowledge.
Giroud scored his first goal since mid-January, while Walcott scored his first goal since December 2015.
Arsenal suffer triple injury blow as they beat Hull City to progress to quarter finals of FA Cup on 8 March.
Aaron Ramsey has a thigh problem but Per Mertesacker and Gabriel should make quick recoveries.
Alex Iwobi has made 10 appearances in all competitions for the Arsenal first-team this season but is yet to score a goal.
Arsenal face Hull City in a fifth round replay after the clash at the Emirates Stadium ended 0-0.
Boss also backed Arsene Wenger despite recent criticism of Gunners' form collapse.
The 28-year-old defender left the Blues to join the French outfit in a big-money move in 2014.
Alex Iwobi to be part of squad for FA Cup fifth round replay at the KC Stadium.
Spaniard heaped praise on the performance of the 18-year-old striker at Old Trafford.
Uruguay defender wanted to leave club after he saw path to the first-team blocked by Darmian and Valencia.
West Ham forward Diafra Sakho has returned to first-team training after being sidelined for three months.
Senegalese midfielder is eligible for selection in the Premier League clash with Sunderland.
Mata's free-kick sent United on their way to a comfortable victory.
The 40-year-old star thinks the Dutchman deserves more time at Old Trafford.
John Terry has missed crucial cup games after suffering a hamstring injury during Chelsea's 5-1 win over Newcastle United.
Wayne Rooney admitted disappointment after missing United's 3-0 win over Shrewsbury due to a knee injury.
United currently have just two fit senior defenders ahead of clashes against Midtjylland and Arsenal.
The inexperienced pair became the latest United players to join their ever-growing injury list.
Centre-back thrived in a defensive midfield role during Saturday's 3-1 FA Cup fifth-round win over West Brom.
Manchester United have to win against Shrewsbury to keep their FA Cup hopes alive.