Unai Emery was appointed as Arsenal head coach in the summer to succEed Arsene Wenger. It took him a bit of time to overhaul the playing squad and get them challenging for the top four and then the Premier League title.
There’s an interesting statistics of how the Spanish tactican had to work his magic of substitutions numerous times in second halves of matches to get a result.
Gunners are currently in their 19-game unbeaten run across all competitions including 4-2 north London derby win over Tottenham Hotspur at the weekend when Emery again used his trick od substitutions.
You can take a look at the five matches this season in which Emery has managed change the game for Arsenal from the bench.
vs Newcastle (September 15, 2018)
Sub: Lucas Torreira for Matteo Guendouzi
Final score: Newcastle 1-2 Arsenal

The Gunners simply struggled in the opening 45 minutes and to his credit, the head coach acknowledged his mistake right at the beginning of the second half and brought on Lucas Torreira for Matteo Guendouzi.
vs Watford (September 29, 2018)
Sub: Alex Iwobi and Danny Welbeck for Aaron Ramsey and Pierre-Emerick Aubameyang
Final score: Arsenal 2-0 Watford

Ramsey had struggled in the number 10 role, while Aubameyang was proving ineffective from the left-wing.
The Welshman was replaced by Iwobi in the 63rd minute and the Nigerian midfielder would go on to force an own goal from Craig Cathcart with a dangerous ball into the area.
vs Leicester City (October 22, 2018)
Sub: Pierre-Emerick Aubameyang for Henrikh Mkhitaryan
Final score: Arsenal 3-1 Leicester City

Taking off Stephan Lichtsteiner and putting Granit Xhaka to left-back turned out to be a masterstroke as was the introduction of Aubameyang in the place of Mkhitaryan.
Within six minutes of coming on the Gabon international had a brace and the hosts were cruising to victory and proving once again that Emery knows when to make a change.
vs Liverpool (November 3, 2018)
Sub: Alex Iwobi for Henrikh Mkhitaryan
Final score: Arsenal 1-1 Liverpool

Replacing Mkhitaryan just seven minutes after James Milner had broken the deadlock at the Emirates, the Nigerian international had a huge hand in Lacazette's equaliser eight minutes from time.
vs Tottenham Hotspur (December 2, 2018)
Subs: Aaron Ramsey and Alexandre Lacazette for Alex Iwobi and Henrikh Mkhitaryan
Final score: Arsenal 4-2 Tottenham Hotspur

Emery made a double change right away meaning Iwobi and Mhkitaryan were the casualties.
On came Ramsey and Lacazette which sparked the turnaround.